In plain words
Better one’s own dharma, though imperfect, than another’s dharma well performed. Performing the action determined by one’s own Svabhāva, one does not incur sin.
Word for word
Better is one's own dharma, imperfect, than another's dharma well performed; doing the duty fixed by svabhāva, one does not incur sin.
The commentators
Bhagavadgītābhāṣyam
यतः एवम् , अतः —
श्रेयान् प्रशस्यतरः स्वो धर्मः स्वधर्मः, विगुणोऽपि इति अपिशब्दो द्रष्टव्यः, परधर्मात् । स्वभावनियतं स्वभावेन नियतम् , यदुक्तं स्वभावजमिति, तदेवोक्तं स्वभावनियतम् इति ; यथा विषजातस्य कृमेः विषं न दोषकरम् , तथा स्वभावनियतं कर्म कुर्वन् न आप्नोति किल्बिषं पापम् ॥ ४७ ॥
In plain words
Because this is how it stands, therefore —
'Śreyān' simply means 'more praiseworthy'; 'svo dharmaḥ' is simply 'svadharma', one's own duty. 'Viguṇo'pi', 'even though flawed' — notice that word 'even': it is doing real work here — better than someone else's duty. 'Svabhāva-niyatam' means fixed by one's own nature — the very thing just called 'svabhāvajam', born of nature, is now called 'svabhāvaniyatam', fixed by nature. Just as poison does no harm to a worm that was born in poison, so a person who does the work fixed by his own nature incurs no sin at all.
Close to the Sanskrit
Since (it is) thus, therefore —
'Śreyān' means more praiseworthy; 'svo dharmaḥ' means 'svadharma'. 'Viguṇo'pi' — the word 'api' is to be noted here — than another's duty. 'Svabhāva-niyatam' means fixed by one's own nature; that which was said, 'svabhāvajam', that very thing is here said as 'svabhāvaniyatam'; just as poison is not harmful to a worm born in poison, so, doing the duty fixed by one's own nature, one does not incur kilbiṣa, that is, sin.