Idiot that man has to cover (6)
I believe the answer is:
clothe
'cover' is the definition.
(synonyms)
'idiot that man' is the wordplay.
'idiot' becomes 'clot' (both can mean a stupid person).
'that man' becomes 'he' (he refers to a man).
'clot'+'he'='CLOTHE'
'has to' is the link.
(Other definitions for clothe that I've seen before include "Dress (someone)" , "Equip" , "Provide cover for body and limbs" , "Invest" , "Put garments onto" .)