Sketch made by doctor at the back of the boat (5)
I believe the answer is:
draft
'sketch' is the definition.
(I know that draft is a type of sketch)
'by doctor at the back of the boat' is the wordplay.
'by' is a charade indicator (letters next to each other).
'doctor' becomes 'Dr' (abbreviation).
'at the back of the boat' becomes 'aft' (I've seen this in another clue).
'dr' next to 'aft' is 'DRAFT'.
'made' acts as a link.
(Other definitions for draft that I've seen before include "sketch, perhaps" , "Rough drawing" , "Preliminary outline" , "Outline or sketch" , "Preliminary drawing" .)