Gloomy when the half moon came up (6)
I believe the answer is:
morose
'gloomy' is the definition.
(I've seen this before)
'the half moon came up' is the wordplay.
'the half' suggests halving the letters.
'came up' becomes 'rose' (I've seen this before).
'moon' cut in half is 'mo'.
'mo'+'rose'='MOROSE'
'when' acts as a link.
(Other definitions for morose that I've seen before include "Gloomy and unsociable" , "Melancholy" , "in gloomy mood" , "Rarely" , "Sullen or glum." .)