Retiring after beginning to appear grey (4)
I believe the answer is:
ashy
'grey' is the definition.
('ashy' can be similar in meaning to 'grey')
'retiring after beginning to appear' is the wordplay.
'retiring' becomes 'shy' (I've seen this in other clues).
'after' says to put letters next to each other.
'beginning to' says to take the initial letters.
The first letter of 'appear' is 'a'.
'shy' after 'a' is 'ASHY'.
(Other definitions for ashy that I've seen before include "Like burnt wood or coal" , "Leaden" , "Like burnt residue" , "resembling cinder track?" , "pale greyish colour" .)