Perfect fish on a lake (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'fish on a lake' is the wordplay.
'fish' becomes 'ide' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'ide'+'a'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "fine" , "Very eager (to see)" , "Standard of perfection or excellence" , "Paradigm" , "Standard to aim for" .)