Draw small boat (6)
I believe the answer is:
'draw' is the definition.
(sketching is a kind of drawing)
'small boat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'boat' becomes 'ketch' (ketch is a kind of sailing vessel).
(Other definitions for sketch that I've seen before include "Rough picture" , "Simple drawing" , "Outline - preliminary drawing" , "An outline drawing or a comedy number" , "Short humorous scene" .)