In opposition to giant as running amok (7)
I believe the answer is:
against
'in opposition to' is the definition.
(I've seen this in another clue)
'giant as running amok' is the wordplay.
'running amok' indicates anagramming the letters.
'giant'+'as'='giantas'
'giantas' is an anagram of 'AGAINST'.
(Other definitions for against that I've seen before include "on the other side" , "Facing" , "Opposite to" , "Not supporting" , "In disagreement with" .)