Small cat is gloomy (6)
I believe the answer is:
smoggy
'gloomy' is the definition.
'small cat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cat' becomes 'moggy' (I've seen this before).
's'+'moggy'='SMOGGY'
'is' acts as a link.
(Other definitions for smoggy that I've seen before include "Hazy, foggy" , "Hazy with pollution" , "Like an unhealthy environment" , "Hazy, smoky" .)