Fish see reflection over lake (4)
I believe the answer is:
eels
'fish' is the definition.
(I've seen this before)
'see reflection over lake' is the wordplay.
'reflection' says the letters should be written backwards.
'over' is an insertion indicator.
'lake' becomes 'l'.
'see' written backwards gives 'ees'.
'ees' going around 'l' is 'EELS'.
(Other definitions for eels that I've seen before include "Devious types" , "Sea creatures found in lees" , "Snakelike fish" , "Abstract nineteen sixties style" , "Sea snakes among lees" .)