Area in Scottish lake for fish (5)
I believe the answer is:
loach
'for fish' is the definition.
(loach is a kind of fish)
'area in scottish lake' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'in' indicates putting letters inside.
'scottish lake' becomes 'loch' (type of body of water).
'a' inserted into 'loch' is 'LOACH'.
(Other definitions for loach that I've seen before include "Fish with barbels around mouth" , "Freshwater fish; rhymes with freshwater fish" , "Film-maker" , "Fish - British film director" , "River fish - film director" .)