So doctor about to crack clue, the last (8)
I believe the answer is:
hindmost
'the last' is the definition.
(I've seen this before)
'so doctor about to crack clue' is the wordplay.
'doctor' becomes 'md' (synonyms).
'about' shows that the letters should be reversed in order.
'to crack' means one lot of letters goes inside another (inserted letters crack into others).
'clue' becomes 'hint' (synonyms).
'so'+'md'='somd'
'somd' written backwards gives 'dmos'.
'dmos' placed within 'hint' is 'HINDMOST'.
(Other definitions for hindmost that I've seen before include "One at the back (devil take him!)" , "after all the others" , "Last" , "Like a tail-ender" , "The one who is furthest behind - devil take him" .)