Opponent in giant, indomitable (4)
I believe the answer is:
Here is my best explanation:
'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 "Opposed to followers of Hitler", "Prefix meaning against", "An opponent - opposed to party policy, etc", "'No' voter", "In opposition to", "Negative vote", "Opponent", "One who is against (something)", "Against, opposed to" and "Policy's opponent".)