When astronomical body going down, doctor ready to act? (7)
I believe the answer is:
moonset
'when astronomical body going down' is the definition.
'doctor ready to act?' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'ready' becomes 'on' (if a device is on it is ready for use).
'to act?' becomes 'set' (set is a kind of act**).
'mo'+'on'+'set'='MOONSET'
(Other definitions for moonset that I've seen before include "when night becomes even darker?" , "astronomical activity" , "Going down of Earth's satellite" , "disappearance of satellite" , "When satellite goes down" .)