Best suit (6)
I believe the answer is:
trumps
'best suit' is the definition.
(card-playing term)
'best suit' is the wordplay.
'best' becomes 'trump' (synonyms).
'suit' becomes 's' (this could be a standard abbreviation which I don't know about).
'trump'+'s'='TRUMPS'
Can you help me to learn more?
(Other definitions for trumps that I've seen before include "could be diamonds" , "Cards designated as having added value in game" , "Playing cards temporarily ranked above others" , "Highest-ranking suit" , ""To come up ___" is to be a great help" .)