Countryman bags his first bird (8)
I believe the answer is:
pheasant
'bird' is the definition.
(I know that pheasant is a type of gallinacean)
'countryman bags his first' is the wordplay.
'countryman' becomes 'peasant'.
'bags' means one lot of letters goes inside another (bag can mean secures or steals).
'his first' becomes 'h' (1st letter of 'his').
'peasant' enclosing 'h' is 'PHEASANT'.
(Other definitions for pheasant that I've seen before include "Wildfowl" , "Large game bird" , "something to pluck?" , "Colourful game bird" , "Game-bird" .)