In conflict with giants - a mess! (7)
I believe the answer is:
against
'in conflict with' is the definition.
(I've seen this before)
'giants a mess' is the wordplay.
'mess' is an anagram indicator.
'giants'+'a'='giantsa'
'giantsa' is an anagram of 'AGAINST'.
(Other definitions for against that I've seen before include "Opposite to" , "Anti, touching" , "Opposing" , "Not in favour of" , "Placed in direct contact with" .)