Fool, big lump (4)
I believe the answer is:
clod
This clue is a double definition.
'fool' is the first definition.
(I've seen this before)
'big lump' is the second definition.
(as in a clod of earth)
(Other definitions for clod that I've seen before include "'Clump, glob (4)'" , "Block" , "Lump of earth or clay" , "dope?" , "Lump of mud" .)