Patron saint of medicine originally involved in new cure (8)
I believe the answer is:
customer
'patron' is the definition.
(I've seen this before)
'saint of medicine originally involved in new cure' is the wordplay.
'saint' becomes 'st' (abbreviation).
'originally' says to take the initial letters.
'involved in' is an insertion indicator.
'new' indicates anagramming the letters.
The first letters of 'of medicine' is 'om'.
'st'+'om'='stom'
'cure' is an anagram of 'cuer'.
'stom' going into 'cuer' is 'CUSTOMER'.
(Other definitions for customer that I've seen before include "Someone shopping" , "Rail passenger, nowadays" , "Patron" , "Person shopping" , "Shopper (or now rail passenger!)" .)