At heart, men's semis test rivals (7)
I believe the answer is:
enemies
'rivals' is the definition.
(enemy is a kind of rival)
'at heart men's semis test' is the wordplay.
I cannot really see how this works, but
an anagram of 'men' is 'nem' which is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for enemies that I've seen before include "Those we fight" , "Foes, adversaries" , "Foes, hostile groups" , "Foes, opponents" .)