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)
'last playing the doctor in' is the wordplay.
'playing' indicates anagramming the letters.
'the doctor' becomes 'mo' (medical officer).
'in' means one lot of letters goes inside another.
'last' is an anagram of 'alst'.
'alst' going around 'mo' is 'ALMOST'.
'the' is the link.
(Other definitions for almost that I've seen before include "Virtually" , "Nearly, not quite" , "Not totally" , "Well-nigh" , "As good as" .)