Clint treated with American medicine (7)
I believe the answer is:
linctus
'medicine' is the definition.
(I've seen this before)
'clint treated with american' is the wordplay.
'treated' is an anagram indicator.
'with' means one lot of letters go next to another.
'american' becomes 'US' (abbreviation for United States).
'clint' anagrammed gives 'linct'.
'linct'+'us'='LINCTUS'
(Other definitions for linctus that I've seen before include "The prescription" , "Cough-medicine" , "Soothing drink" , "Thick liquid medicine" , "Special formulation" .)