A small puff - blue (3)
I believe the answer is:
sad
'blue' is the definition.
(similar in meaning)
'a small puff' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'puff' becomes 'ad' (short for advert - a puff piece is an advertisemen).
's'+'ad'='SAD'
(Other definitions for sad that I've seen before include "Miserable" , "Mournful - sorry" , "Mournful and depressed" , "Regrettable" , "Gloomy" .)