Creature fell finally during hostilities with America (6)
I believe the answer is:
walrus
'creature' is the definition.
'fell finally during hostilities with america' is the wordplay.
'finally' indicates one should take the final letters.
'during' indicates putting letters inside.
'hostilities' becomes 'war' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'america' becomes 'us' (abbreviation for the United States**).
The final letter of 'fell' is 'l'.
'l' put within 'war' is 'walr'.
'walr'+'us'='WALRUS'
(Other definitions for walrus that I've seen before include "'The time has come', the . . . said" , "Aquatic mammal" , "Marine mammal with tusks and a tough hide" , "Large tusked marine mammal" , "Marine mammal with two large tusks" .)