After ten rounds men part, resorting to an underhand tactic (10)
I believe the answer is:
entrapment
'an underhand tactic' is the definition.
(tricking someone into committing a crime)
'after ten rounds men part resorting' is the wordplay.
'after' is a charade indicator (letters next to each other).
'rounds' indicates an anagram (I've seen 'round' mean this).
'resorting' is an anagram indicator.
'ten' with letters rearranged gives 'ent'.
'men'+'part'='menpart'
'menpart' with letters rearranged gives 'rapment'.
'ent' next to 'rapment' is 'ENTRAPMENT'.
'to' acts as a link.
(Other definitions for entrapment that I've seen before include "Sting" , "Tricking (of someone) into crime" , "(Act of) tricking someone into getting caught" , "Pet remnant (anag.)" .)