Large dons mock idiot (4)
I believe the answer is:
clod
'idiot' is the definition.
(thesaurus)
'large dons mock' is the wordplay.
'large' becomes 'l'.
'dons' is an insertion indicator (don means to wear or put on).
'mock' becomes 'cod' (codding is a kind of mocking).
'l' going within 'cod' is 'CLOD'.
(Other definitions for clod that I've seen before include "Lump of mud" , "Silly person" , "Compact mass - stupid person" , "Fool - bit of turf" , "Stupid person - compact lump" .)