Mint coin — sovereign (10)
I believe the answer is:
pennyroyal
'mint' is the definition.
(pennyroyal is a kind of mint)
'coin sovereign' is the wordplay.
'coin' becomes 'penny' (penny is a kind of coin).
'sovereign' becomes 'royal' (I've seen this before**).
'penny'+'royal'='PENNYROYAL'
(Other definitions for pennyroyal that I've seen before include "Plant used in herbal medicine (part of the queen's shilling?)" , "medicinal plant" , "Small plant of mint family" .)