Sailor rested, having crossed lake (4)
I believe the answer is:
salt
'sailor' is the definition.
(salt is a kind of sailor)
'rested having crossed lake' is the wordplay.
'rested' becomes 'sat' (to sit is to rest).
'having crossed' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'sat' placed around 'l' is 'SALT'.
(Other definitions for salt that I've seen before include "Crystalline substance used to season food" , "Crystalline substance for the table" , "Condiment - experienced sailor" , "Condiment - old sailor" , "Preserve" .)