Lump fish circling loch (4)
I believe the answer is:
clod
'lump' is the definition.
(synonyms)
'fish circling loch' is the wordplay.
'fish' becomes 'cod' (type of fish).
'circling' is an insertion indicator (circling can mean going around or containing).
'loch' becomes 'l'.
'cod' enclosing 'l' is 'CLOD'.
(Other definitions for clod that I've seen before include "Lump of earth or clay" , "Clump - chump" , "Lump of earth/clay" , "Hunk" , "Fool - bit of turf" .)