Greek character boxing a marine shows revulsion (6)
I believe the answer is:
nausea
'revulsion' is the definition.
(I've seen this in another clue)
'greek character boxing a marine' is the wordplay.
'greek character' becomes 'nu' (letter from the Greek alphabet).
'boxing' is an insertion indicator ('box' can mean contain, as in to put in a box).
'marine' becomes 'sea' (both can mean 'of the sea').
'nu' enclosing 'a' is 'nau'.
'nau'+'sea'='NAUSEA'
'shows' is the link.
(Other definitions for nausea that I've seen before include "Feeling of imminent sickness or upset stomach" , "Sensation of queasiness" , "Malady" , "A feeling of disgust or illness" , "Ill-feeling" .)