Grey area likely to shrink (4)
I believe the answer is:
ashy
'grey' is the definition.
(similar in meaning)
'area likely to shrink' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'likely to shrink' becomes 'shy' (I've seen this in another clue).
'a'+'shy'='ASHY'
(Other definitions for ashy that I've seen before include "Grey-coloured" , "Light grey" , "Wan" , "Like burnt remains" , "Like burnt wood or coal" .)