Doctor working to reveal buttocks (4)
I believe the answer is:
moon
'to reveal buttocks' is the definition.
(to moon someone is to show them your buttocks)
'doctor working' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'working' becomes 'on' (on can mean working or activated).
'mo'+'on'='MOON'
(Other definitions for moon that I've seen before include "Satellite revolving round the earth" , "Remove boxers, maybe" , "Appear listless" , "Satellite of six Down" , "Expose one's buttocks" .)