Unrivalled learner in dull board game (9)
I believe the answer is:
matchless
'unrivalled' is the definition.
(synonyms)
'learner in dull board game' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' is an insertion indicator.
'dull' becomes 'mat' (mat as opposed to glossy).
'board game' becomes 'chess' (chess is a kind of board game).
'mat'+'chess'='matchess'
'l' put within 'matchess' is 'MATCHLESS'.
(Other definitions for matchless that I've seen before include "Having no equal" , "Unique" , "Unrivalled" , "Unparalleled" , "Unbeatable" .)