Ultimate aid for swimmer on a lake (5)
I believe the answer is:
final
'ultimate' is the definition.
(similar in meaning)
'aid for swimmer on a lake' is the wordplay.
'aid for swimmer' becomes 'fin' (fish use fins to swim).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'lake' becomes 'L' (geographical abbreviation).
'fin'+'a'+'l'='FINAL'
(Other definitions for final that I've seen before include "The last exam or leg of competition" , "Last match in a competition" , "Not to be altered" , "Last, ultimate" , "binding" .)