When heavenly body drops idle god (7)
I believe the answer is:
moonset
'when heavenly body drops' is the definition.
(when the moon sets)
'idle god' is the wordplay.
'idle' becomes 'moon' (mooning is a kind of idling).
'god' becomes 'set' (Egyptian god).
'moon'+'set'='MOONSET'
(Other definitions for moonset that I've seen before include "when night becomes even darker?" , "Going down of Earth's satellite" , "When Earth's satellite disappears below the horizon" , "disappearance of satellite" , "astronomical activity" .)