Gamekeeper’s beginning to stir up birds (6)
I believe the answer is:
grouse
'birds' is the definition.
'gamekeeper's beginning to stir up' is the wordplay.
'beginning to' says to take the initial letters.
'stir up' becomes 'rouse' (I've seen this before).
The first letter of 'gamekeeper' is 'g'.
'g'+'rouse'='GROUSE'
(Other definitions for grouse that I've seen before include "Gripe; game bird" , "The complaint of a gamebird" , "Petty complaint" , "Complain pettily" , "See game bird" .)