Fish one found in lake (5)
I believe the answer is:
loach
'fish' is the definition.
(loach is a kind of fish)
'one found in lake' is the wordplay.
'one' becomes 'a'.
'found in' means one lot of letters goes inside another.
'lake' becomes 'loch' (loch is a kind of lake).
'a' going within 'loch' is 'LOACH'.
(Other definitions for loach that I've seen before include "River fish - film director" , "Fish with barbels around mouth" , "Freshwater fish; rhymes with freshwater fish" , "Fish - British film director" , "Film-maker" .)