In plain words
But a yajña performed with a view to its fruit or for display, best of Bharatas, know that yajña to be rājasa.
Word for word
But what is offered aiming at fruit, and also indeed for display, O best of the Bhāratas, know that yajña [to be] rājasa.
अभिसन्धाय तु फलं दम्भार्थमपि चैव यत्इज्यते भरतश्रेष्ठ तं यज्ञं विद्धि राजसम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
अभिसन्धाय तु उद्दिश्य फलं दम्भार्थमपि चैव यत् इज्यते भरतश्रेष्ठ तं यज्ञं विद्धि राजसम् ॥ १२ ॥
In plain words
But a sacrifice offered with the payoff already in mind — abhisandhāya, aiming at it, singling it out — and offered for show as well: know that sacrifice, best of the Bharatas, as rājasic.
Close to the Sanskrit
But that which is offered having aimed at — that is, having in view — a fruit, and also indeed for the sake of ostentation: that sacrifice, O best of the Bharatas, know to be rājasic.