Flier possibly cruel at the Western Front (6)
I believe the answer is:
curlew
'flier' is the definition.
(curlew is a kind of birdanimal)
'possibly cruel at the western front' is the wordplay.
'possibly' indicates an anagram.
'at' is a charade indicator (letters next to each other).
'front' suggests taking the first letters.
The initial letter of 'western' is 'w'.
'cruel' is an anagram of 'curle'.
'curle'+'w'='CURLEW'
(Other definitions for curlew that I've seen before include "Large shorebird" , "Flyer" , "Large wading bird" , "Wading bird with long slender curved bill" , "Wading bird with long down-curved beak" .)