Lotion used in herbal medicine (4)
I believe the answer is:
balm
'lotion' is the definition.
(I've seen this in another clue)
'in herbal medicine' is the wordplay.
'in' indicates the answer is hidden within the clue.
'BALM' can be found hidden inside 'herbal medicine'.
'used' is the link.
(Other definitions for balm that I've seen before include "'Ointment, salve (4)'" , "Unguent" , "Soothing stuff for lamb" , "Soothing ointment for lamb (4)" , "skin cream" .)