A hen’s apt to be mistaken for a game bird (8)
I believe the answer is:
pheasant
'for a game bird' is the definition.
(pheasant is a kind of game bird)
'a hen's apt to be mistaken' is the wordplay.
'to be mistaken' is an anagram indicator.
'a'+'hens'+'apt'='ahensapt'
'ahensapt' is an anagram of 'PHEASANT'.
(Other definitions for pheasant that I've seen before include "Long-tailed gamebirds, male has bright feathers" , "Ant heaps (anag.)" , "Wildfowl" , "Colourful game bird" , "something to pluck?" .)