Allowed on a lake (5)
I believe the answer is:
legal
'allowed' is the definition.
(I know that allowed can be written as legal)
'on a lake' is the wordplay.
'on' becomes 'leg' (cricket term, fielding positions).
'lake' becomes 'L' (geographical abbreviation).
'leg'+'a'+'l'='LEGAL'
(Other definitions for legal that I've seen before include "Like some tender" , "Licit" , "within the rules" , "In accordance with law" , "Conforming with the law" .)