A small notice in blue (3)
I believe the answer is:
sad
'blue' is the definition.
(I know that blue can be written as sad)
'a small notice' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'notice' becomes 'ad' (both can mean an advertisement).
's'+'ad'='SAD'
'in' is the link.
(Other definitions for sad that I've seen before include "Regrettable" , "Doleful" , "Morose" , "A form of depression in winter, initially" , "Dismal" .)