A dunce back to being distant (5)
I believe the answer is:
aloof
'distant' is the definition.
(synonyms)
'a dunce back' is the wordplay.
'dunce' becomes 'fool' (both can mean a stupid person).
'back' says the letters should be written in reverse.
'fool' reversed gives 'loof'.
'a'+'loof'='ALOOF'
'to being' is the link.
(Other definitions for aloof that I've seen before include "rather cold" , "Indifferent - by oneself" , "Withdrawn, distant" , "Detached, distant" , "reserved" .)