Opponent of slack stagnation (10)
I believe the answer is:
antagonist
'opponent' is the definition.
(synonyms)
'slack stagnation' is the wordplay.
'slack' indicates anagramming the letters (a slack or relaxed spelling of the letters).
'stagnation' is an anagram of 'ANTAGONIST'.
'of' acts as a link.
(Other definitions for antagonist that I've seen before include "5 down (anag)" , "Adversary" , "One who is hostile" , "rival" , "Opponent" .)