Outline of small boat (6)
I believe the answer is:
sketch
'outline' is the definition.
('sketch' can be a synonym of 'outline')
'small boat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'boat' becomes 'ketch' (ketch is a kind of sailing vessel).
's'+'ketch'='SKETCH'
'of' acts as a link.
(Other definitions for sketch that I've seen before include "Slow of understanding" , "Brief outline or drawing" , "Short comic scene" , "Simple drawing" , "Short dramatic scene" .)