Mineral used in dental clinics (4)
I believe the answer is:
'mineral' is the definition.
(talc is a kind of mineral)
'in dental clinics' is the wordplay.
'in' indicates a hidden word.
'TALC' is hidden within 'dental clinics'.
'used' is the link.
(Other definitions for talc that I've seen before include "In brief, popular body powder" , "Magnesium silicate powder" , "Bathroom dusting powder" , "Perfumed drying powder" , "Softest substance in Mohs scale of hardness" .)