Foul relative has a name (7)
I believe the answer is:
unclean
'foul' is the definition.
(similar in meaning)
'relative has a name' is the wordplay.
'relative' becomes 'uncle' (uncle is a kind of relative).
'has' says to put letters next to each other.
'name' becomes 'n' (common abbreviation e.g. on forms).
'uncle'+'a'+'n'='UNCLEAN'
(Other definitions for unclean that I've seen before include "Spotted" , "Soiled like an uncle" , "Ceremonially impure" , "Defiled" , "Dirty, needs a wash" .)