Shabbily dressed in blue, slightly behind (4-2-4)
I believe the answer is:
down-at-heel
'shabbily dressed' is the definition.
'blue slightly behind' is the wordplay.
'blue' becomes 'down' (both can mean sad).
'slightly behind' becomes 'at heel' (as in a dog coming at heel).
'down'+'atheel'='DOWN-AT-HEEL'
'in' is the link.
(Other definitions for down-at-heel that I've seen before include "In poor circumstances" , "Having a poor, shabby appearance" , "Generally shabby" , "Looking shabby" .)