Doctor of mercy to get medicinal herb (7)
I believe the answer is:
comfrey
'medicinal herb' is the definition.
(I've seen this in another clue)
'doctor of mercy' is the wordplay.
'doctor' indicates an anagram.
'of'+'mercy'='ofmercy'
'ofmercy' with letters rearranged gives 'COMFREY'.
'to get' is the link.
(Other definitions for comfrey that I've seen before include "Hairy-leaved plant" , "Hairy-leaf, medicinal herb" .)