Unrivalled when not getting a game (9)
I believe the answer is:
matchless
'unrivalled' is the definition.
(synonyms)
'not getting a game' is the wordplay.
'not' becomes 'less' (I've seen this in other clues).
'getting' means one lot of letters go next to another.
'a game' becomes 'match' (eg a football match).
'less' put after 'match' is 'MATCHLESS'.
'when' is the link.
(Other definitions for matchless that I've seen before include "Perfect" , "unlike Andersen's poor little girl?" , "Without peer, nonpariel" , "Unique" , "Unparalleled" .)