Opponents in game involved in sequence played with different sides (7)
I believe the answer is:
scalene
'played with different sides' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs can sometimes define each other.
'opponents in game involved in sequence' is the wordplay.
'opponents' becomes 'EN' (East and North players in the card game bridge).
'in' is an insertion indicator.
'game involved in sequence' becomes 'scale' (I am not sure about this - if you are sure you should believe this answer much more).
'en' put within 'scale' is 'SCALENE'.
Can you help me to learn more?
(Other definitions for scalene that I've seen before include "(Of triangle) having three unequal sides" , "(Of triangle) with each side of a different length" , "not having evenly matched sides" , "(Of triangle) with no sides equal" , "Having no sides for a match" .)