Fake flier -- never mind! (6,2)
I believe the answer is:
forget it
'never mind' is the definition.
(thesaurus)
'fake flier' is the wordplay.
'fake' becomes 'forge' ('forge' can be a synonym of 'fake').
'flier' becomes 'tit' (tit is a kind of birdanimal**).
'forge'+'tit'='FORGET IT'
(Other definitions for forget it that I've seen before include "Let it go!" , "Not a chance" , "There's no need for apologies" , "ignore something" , "Think no more of the matter" .)