Young eel abandoning lake at any time (4)
I believe the answer is:
ever
'at any time' is the definition.
(synonyms)
'young eel abandoning lake' is the wordplay.
'young eel' becomes 'elver' (an elver is a young eel).
'abandoning' is a deletion indicator.
'lake' becomes 'l' (geographical abbreviation).
'elver' with 'l' taken out is 'EVER'.
(Other definitions for ever that I've seen before include "Always, at any time" , "Veer away endlessly" , "Always and permanent" , "To the end of time" , "Possibly" .)