Doctor into rock as solo player (5)
I believe the answer is:
ombre
'solo player' is the definition.
(term for a player in the card game ombre)
'doctor into rock' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine, Medicinae Baccalaureus).
'into' is an insertion indicator.
'rock' becomes 'ore' (ores are found in rocks).
'mb' going inside 'ore' is 'OMBRE'.
'as' acts as a link.
(Other definitions for ombre that I've seen before include "Shaded" , "solo player" , "Tricky game" , "Game using 40 cards" , "With blending colours" .)