Grey area's shrinking (4)
I believe the answer is:
ashy
'grey' is the definition.
(similar in meaning)
'area's shrinking' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'shrinking' becomes 'shy' (I've seen this in another clue).
'a'+'shy'='ASHY'
(Other definitions for ashy that I've seen before include "Like, eg, volcanic dust" , "covered in dust" , "Light grey" , "What could tray be" , "Like fireplace residue" .)