Some frantic person opposing something (4)
I believe the answer is:
Here is my best explanation:
'person opposing something' is the definition.
'some frantic' is the subsidiary indication.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'anti' is hidden in the letters of 'frantic'.
(Other definitions for anti that I've seen before include "Naysayer", "Not in favor of", "Hostile", "on the other side", "Negative vote", "not favoured", "Opposed to followers of Hitler", "An opponent - opposed to party policy, etc", "Policy's opponent" and "One opposing".)