Giants moan about hostility (10)
I believe the answer is:
antagonism
'hostility' is the definition.
(I know that antagonism is a type of hostility)
'giants moan about' is the wordplay.
'about' indicates anagramming the letters.
'giants'+'moan'='giantsmoan'
'giantsmoan' with letters rearranged gives 'ANTAGONISM'.
(Other definitions for antagonism that I've seen before include "Active hostility" , "Active opposition" .)