Something deemed medicinal I understand and must swallow (7)
I believe the answer is:
aniseed
'something deemed medicinal' is the definition.
(anise is used in herbal medicines)
'i understand and must swallow' is the wordplay.
'understand' becomes 'see' (synonyms).
'must swallow' indicates putting letters inside.
'i'+'see'='isee'
'isee' put within 'and' is 'ANISEED'.
(Other definitions for aniseed that I've seen before include "Flavouring herb" , "Liquorice flavour" , "Cake flavouring" , "Aromatic flavouring" , "Liquorice flavouring, used in cookery" .)