Very large turkey to begin with, full-flavoured bird (7)
I believe the answer is:
ostrich
'bird' is the definition.
(ostrich is a kind of bird)
'very large turkey to begin with full-flavoured' is the wordplay.
'very large' becomes 'os' (abbreviation for oversize).
'to begin with' suggests taking the first letters.
'full-flavoured' becomes 'rich' (I've seen this before).
The first letter of 'turkey' is 't'.
'os'+'t'+'rich'='OSTRICH'
(Other definitions for ostrich that I've seen before include "Nut-burying type" , "Creature" , "source of showy feathers" , "Rich sot seeks large bird" , "One refusing to face facts; flightless African bird" .)