Herb used in frothy medicine (5)
I believe the answer is:
thyme
'herb' is the definition.
(thyme is a kind of herb)
'in frothy medicine' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THYME' is hidden in the letters of 'frothy medicine'.
'used' acts as a link.
(Other definitions for thyme that I've seen before include "Culinary herb of the mint family" , "Aromatic ingredient" , "Sweet-smelling herb of the mint family" , "Strong-scented herb" , "Meadow herb" .)