Bad giant's action is hostile (12)
I believe the answer is:
antagonistic
'hostile' is the definition.
('antagonistic' can be similar in meaning to 'hostile')
'bad giant's action' is the wordplay.
'bad' indicates an anagram.
'giants'+'action'='giantsaction'
'giantsaction' anagrammed gives 'ANTAGONISTIC'.
'is' acts as a link.
(Other definitions for antagonistic that I've seen before include "Actively opposing - stag in action (anag)" , "Ill-disposed" , "Actively hostile" .)