All but the last, playing the doctor in (6)
I believe the answer is:
almost
'all but' is the definition.
(I've seen this before)
'the last playing the doctor in' is the wordplay.
'playing' is an anagram indicator.
'the doctor' becomes 'mo' (medical officer).
'in' means one lot of letters goes inside another.
'last' with letters rearranged gives 'alst'.
'alst' going around 'mo' is 'ALMOST'.
(Other definitions for almost that I've seen before include "Virtually" , "Nearly, not quite" , "Not totally" , "Well-nigh" , "As good as" .)