Grey area reserved (4)
I believe the answer is:
ashy
'grey' is the definition.
('ashy' can be similar in meaning to 'grey')
'area reserved' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'reserved' becomes 'shy' (I've seen this in other clues).
'a'+'shy'='ASHY'
(Other definitions for ashy that I've seen before include "Cinders must have been this" , "Leaden" , "Like burnt remains" , "evidence of recent fire" , "Dusty" .)