Stupid giant's action showing opposition (12)
I believe the answer is:
antagonistic
'opposition' is the definition.
(thesaurus)
'stupid giant's action' is the wordplay.
'stupid' indicates anagramming the letters (in a stupid order).
'giants'+'action'='giantsaction'
'giantsaction' with letters rearranged gives 'ANTAGONISTIC'.
'showing' is the link.
(Other definitions for antagonistic that I've seen before include "Actively opposing - stag in action (anag)" , "Actively hostile" , "Ill-disposed" .)