Sailor perched when traversing lake (4)
I believe the answer is:
salt
'sailor' is the definition.
(salt is a kind of sailor)
'perched when traversing lake' is the wordplay.
'perched' becomes 'sat' (I've seen this before).
'when traversing' means one lot of letters goes inside another (traverse can mean to go around).
'lake' becomes 'L' (geographical abbreviation).
'sat' going around 'l' is 'SALT'.
(Other definitions for salt that I've seen before include "Crystalline substance for the table" , "Crystalline substance used to season food" , "Common substance" , "Condiment - old sailor" , "Brackish" .)