Doctor on satellite (4)
I believe the answer is:
moon
'satellite' is the definition.
(I know that moon is a type of satellite)
'doctor on' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'mo'+'on'='MOON'
(Other definitions for moon that I've seen before include "who headed a religious 29 [CULT]" , "Display one's buttocks publicly (slang)" , "Reveal rear" , "Remove boxers, maybe" , "Orbiting body" .)