Unusually cruel start to war for flier (6)
I believe the answer is:
curlew
'flier' is the definition.
(curlew is a kind of birdanimal)
'unusually cruel start to war' is the wordplay.
'unusually' is an anagram indicator.
'start to' says to take the initial letters.
The first letter of 'war' is 'w'.
'cruel' with letters rearranged gives 'curle'.
'curle'+'w'='CURLEW'
'for' acts as a link.
(Other definitions for curlew that I've seen before include "Bird with long down-curved bill" , "Shorebird with long curved bill" , "Large wader of the sandpiper family" , "Wader with curved bill" , "Wading bird with long down-curved beak" .)