Hostile to giant as let loose (7)
I believe the answer is:
against
'hostile to' is the definition.
(I've seen this in another clue)
'giant as let loose' is the wordplay.
'let loose' indicates anagramming the letters.
'giant'+'as'='giantas'
'giantas' anagrammed gives 'AGAINST'.
(Other definitions for against that I've seen before include "close by" , "In opposition to" , "Versus - in contact with" , "In contrast to; touching" , "V" .)