Powder used in dental clinics (4)
I believe the answer is:
talc
'powder' is the definition.
(I know that talc is a more specific form of the action powder)
'in dental clinics' is the wordplay.
'in' indicates a hidden word.
'TALC' can be found hidden inside 'dental clinics'.
'used' is the link.
(Other definitions for talc that I've seen before include "Soft mineral; powder" , "Softest mineral on Mohs hardness scale" , "Powdered substance" , "Soft white powder" , "Powdered mineral" .)