Retiring after one becomes grey (4)
I believe the answer is:
ashy
'grey' is the definition.
(similar in meaning)
'retiring after one' is the wordplay.
'retiring' becomes 'shy' (I've seen this before).
'after' says to put letters next to each other.
'one' becomes 'a'.
'shy' put after 'a' is 'ASHY'.
'becomes' is the link.
(Other definitions for ashy that I've seen before include "Deathly pale" , "Like burnt residue" , "Like, eg, volcanic dust" , "Like burnt remains" , "Dusty" .)