Reserved area idiot rejected (5)
I believe the answer is:
aloof
'reserved' is the definition.
(similar in meaning)
'area idiot rejected' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'idiot' becomes 'fool' (I've seen this before).
'rejected' shows that the letters should be reversed in order.
'fool' in reverse letter order is 'loof'.
'a'+'loof'='ALOOF'
(Other definitions for aloof that I've seen before include "Blood deficiency" , "Distant, uninvolved" , "Coldly distant" , "apart from all else" , "Cool and remote, even unfriendly" .)