Doctor with practice on computer accessory (5)
I believe the answer is:
mouse
'computer accessory' is the definition.
(a computer mouse)
'doctor with practice' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'with' means one lot of letters go next to another.
'practice' becomes 'use' (use is a kind of practice).
'mo'+'use'='MOUSE'
'on' acts as a link.
(Other definitions for mouse that I've seen before include "One sitting by computer" , "whose movements are captured on screen" , "Small rodent - computer attachment" , "Computer accessory" , "Small verminous rodent" .)