Oil found in herbal medicine (4)
I believe the answer is:
balm
'oil' is the definition.
(thesaurus)
'in herbal medicine' is the wordplay.
'in' says the answer is hidden in the clue.
'BALM' is hidden within 'herbal medicine'.
'found' acts as a link.
(Other definitions for balm that I've seen before include "It comforts, soothes" , "Fragrant and healing ointment" , "It's comforting" , "'Ointment, salve (4)'" , "Aromatic herb" .)