A youth going on lake for sail (6)
I believe the answer is:
lateen
'for sail' is the definition.
(lateen sail is a kind of sail)
'a youth going on lake' is the wordplay.
'youth' becomes 'teen' (teenager).
'going on' says to put letters next to each other.
'lake' becomes 'L' (geographical abbreviation).
'a'+'teen'='ateen'
'ateen' after 'l' is 'LATEEN'.
(Other definitions for lateen that I've seen before include "sailing boat" , "Rigged with triangle sail; rhymes with glossy cloth" , "Triangular sail" .)