In plain words
But action performed with great strain by one who desires its result or acts with ahaṅkāra is declared rājasa.
Word for word
But which action is done by one seeking what he desires, or again by one with ahaṅkāra, with much strain, that is declared rājasa.
यत् तु कामेप्सुना कर्म साहङ्कारेण वा पुनःक्रियते बहुलायासं तद् राजसमुदाहृतम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
यत्तु कामेप्सुना कर्मफलप्रेप्सुना इत्यर्थः, कर्म साहङ्कारेण इति न तत्त्वज्ञानापेक्षया । किं तर्हि ? लौकिकश्रोत्रियनिरहङ्कारापेक्षया । यो हि परमार्थनिरहङ्कारः आत्मवित् , न तस्य कामेप्सुत्वबहुलायासकर्तृत्वप्राप्तिः अस्ति । सात्त्विकस्यापि कर्मणः अनात्मवित् साहङ्कारः कर्ता, किमुत राजसतामसयोः । लोके अनात्मविदपि श्रोत्रियो निरहङ्कारः उच्यते ‘निरहङ्कारः अयं ब्राह्मणः’ इति । तस्मात् तदपेक्षयैव ‘साहङ्कारेण वा’ इति उक्तम् । पुनःशब्दः पादपूरणार्थः । क्रियते बहुलायासं कर्त्रा महता आयासेन निर्वर्त्यते, तत् कर्म राजसम् उदाहृतम् ॥ २४ ॥
In plain words
As for 'by one desiring out of desire' — this means 'by one desiring the fruit of action'. And when the verse says action is done 'with egoism', that is not measured against philosophical Self-knowledge. Against what, then? Against the ordinary sense of 'egoless', the kind applied to a learned scholar in everyday speech. Because for someone who is truly egoless — the actual knower of the Self — becoming a desire-driven, effortful agent at all simply isn't possible for him. In fact, even performing sāttvika action, a person who hasn't realized the Self remains an egoistic agent; how much more so with rajasic or tamasic action! In everyday speech, even someone who hasn't realized the Self gets called 'egoless' if he's simply learned — as when people say, 'this brahmin is egoless.' That is exactly the ordinary sense the verse relies on when it says 'with egoism, or'. The word 'again' here is purely metrical, just filling out the line. 'Done with great exertion, carried out through great effort by the agent' — that kind of action is cited as an example of the rajasic type.
Close to the Sanskrit
As for 'by one desiring out of desire' — 'by one desiring the fruit of action', this is the meaning; 'action', 'with egoism' — thus, this is not with reference to true knowledge. What then? With reference to the egolessness of an ordinary learned man. For, whoever is egoless in the ultimate sense, the knower of the Self — for him, the attainment of being an agent driven by desire and great effort does not exist. Even of sāttvika action, a non-knower of the Self is an egoistic agent; how much more so of rajasic and tamasic action! In the world, even a non-knower of the Self, if learned, is called 'egoless' — 'this brahmin is egoless', thus. Therefore, with reference to that very everyday sense, 'with egoism, or', thus, it was said. The word 'punaḥ' has the purpose of filling out the metrical foot. 'Is done with great exertion, is accomplished by the agent, by great effort' — that action is exemplified as rajasic.