Idiot nicking empty space in cupboard (6)
I believe the answer is:
closet
'cupboard' is the definition.
(synonyms)
'idiot nicking empty space' is the wordplay.
'idiot' becomes 'clot' (I've seen this in other clues).
'nicking' means one lot of letters goes inside another (I've seen this in other clues).
'empty' says to hollow out the word (remove centre letters).
'space' with its centre removed is 'se'.
'clot' placed around 'se' is 'CLOSET'.
'in' acts as a link.
(Other definitions for closet that I've seen before include "place to store gear" , "Furtive" , "Covert" , "A small room or cupboard" , "Secret - cupboard" .)