Fish swallowing large lump of earth (4)
I believe the answer is:
clod
'lump of earth' is the definition.
(I've seen this before)
'fish swallowing large' is the wordplay.
'fish' becomes 'cod' (type of fish).
'swallowing' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'cod' enclosing '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" .)