Solitary record success includes REM riff (6)
I believe the answer is:
hermit
'solitary' is the definition.
(a solitary person)
'record success includes rem riff' is the wordplay.
'record success' becomes 'hit' (hit record).
'includes' is an insertion indicator.
'riff' indicates anagramming the letters (a riff or improvised form based on the letters).
'rem' with letters rearranged gives 'erm'.
'hit' enclosing 'erm' is 'HERMIT'.
(Other definitions for hermit that I've seen before include "Anchorite, person who has withdrawn from society" , "Person like Peter" , "Anchorite, one who lives alone for religious reasons" , "Recluse, one who lives in solitude" , "Eremite" .)