Fellow translated only the first three in sacred text (6)
I believe the answer is:
mantra
'sacred text' is the definition.
(mantra is a kind of sacred text)
'fellow translated only the first three in' is the wordplay.
'fellow' becomes 'man' (I've seen this before).
'translated only' becomes 'ra' (I can't explain this - if you can you should give a lot more credence to this answer).
'the first three' becomes 't' (rst letter of 'three').
'in' indicates putting letters inside.
'man'+'ra'='manra'
'manra' enclosing 't' is 'MANTRA'.
(Other definitions for mantra that I've seen before include "Commonly repeated word or phrase, with religious connotation perhaps" , "Oft-repeated slogan or phrase" , "Repeated sacred word or words" , "Phrase repeated as aid to meditation in Buddhism" , "repeating phrase" .)