Opponent in giant, indomitable (4)
I believe the answer is:
anti
'opponent' is the definition.
(I've seen this before)
'in giant indomitable' is the wordplay.
'in' says the answer is hidden in the clue.
'ANTI' can be found hidden inside 'giant indomitable'.
(Other definitions for anti that I've seen before include "(Person who is) not in favour" , "Prefix meaning against" , "Who pooh-poohs all" , "Someone opposed (to a policy?)" , "Definitely not pro" .)