Fish swallowing large lump of earth (4)
I believe the answer is:
'lump of earth' is the definition.
(I've seen this before)
'fish swallowing large' is the wordplay.
'fish' becomes 'cod' (type of fish).
'swallowing' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'cod' going around 'l' is 'CLOD'.
(Other definitions for clod that I've seen before include "Silly person" , "Stupid person - compact lump" , "Hunk" , "Fool - lump of earth" , "Clump - chump" .)