Outline of second sailing boat (6)
I believe the answer is:
sketch
'outline' is the definition.
(I know that outline can be written as sketch)
'second sailing boat' is the wordplay.
'second' becomes 's' (common abbreviation).
'sailing boat' becomes 'ketch' (type of sailing boat).
's'+'ketch'='SKETCH'
'of' is the link.
(Other definitions for sketch that I've seen before include "Rapid drawing - short play" , "Rough picture" , "Slow of understanding" , "Rough outline" , "Short humorous scene" .)