Smiling primarily, except when gloomy (7)
I believe the answer is:
sunless
'gloomy' is the definition.
(I've seen this before)
'smiling primarily except' is the wordplay.
'primarily' suggests taking the first letters.
'except' becomes 'unless' (I've seen this before).
The initial letter of 'smiling' is 's'.
's'+'unless'='SUNLESS'
'when' acts as a link.
(Other definitions for sunless that I've seen before include "Unlit by solar light" , "Clouded over" , "gloomy" , "grey" , "dark" .)