Feature of vessel on lake is excellent, excellent! (7)
I believe the answer is:
topsail
'feature of vessel' is the definition.
(type of sail)
'on lake is excellent excellent' is the wordplay.
'on' is a charade indicator (letters next to each other).
'lake' becomes 'l'.
'is excellent' becomes 'tops' (I've seen this before).
'excellent' becomes 'ai' (resembles 'At', old-fashioned term for excellent).
'tops'+'ai'='topsai'
'l' after 'topsai' is 'TOPSAIL'.
(Other definitions for topsail that I've seen before include "Highly-placed canvas" , "Upper piece of ship's canvas" , "Part of ship's canvas" , "It's set high up on a sailing ship" , "sheet of canvas" .)