Hostile as giant runs amok (7)
I believe the answer is:
against
'hostile' is the definition.
(thesaurus)
'as giant runs amok' is the wordplay.
'runs amok' indicates anagramming the letters.
'as'+'giant'='asgiant'
'asgiant' with letters rearranged gives 'AGAINST'.
(Other definitions for against that I've seen before include "close by" , "Abutting" , "Facing" , "Placed in direct contact with" , "Versus - in contact with" .)