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 subsidiary indication.
'in' indicates the answer is hidden within the clue.
'anti' is hidden within 'giant indomitable'.
(Other definitions for anti that I've seen before include "In opposition to", "Not in favour", "Not in favor of", "Against, opposed to", "Prefix meaning against", "Policy's opponent", "Negative vote", "An opponent - opposed to party policy, etc", "Opponent" and "Opposed to followers of Hitler".)