I fight in engagement that’s unequal (9)
I believe the answer is:
disparate
'unequal' is the definition.
(I've seen this in another clue)
'i fight in engagement' is the wordplay.
'fight' becomes 'spar' (sparring is a kind of fighting).
'in' means one lot of letters goes inside another.
'engagement' becomes 'date' (synonyms).
'i'+'spar'='ispar'
'ispar' placed within 'date' is 'DISPARATE'.
'that's' is the link.
(Other definitions for disparate that I've seen before include "Contrasting" , "Quite different" , "Including elements very different from one another" .)