Italian dukes once ruled here, provided priest turned up (6)
I believe the answer is:
amalfi
'italian dukes once ruled here' is the definition.
'provided priest turned up' is the wordplay.
'provided' becomes 'if'.
'priest' becomes 'lama' (lama is a kind of priest).
'turned up' says the letters should be written in reverse.
'if'+'lama'='iflama'
'iflama' back-to-front is 'AMALFI'.
(Other definitions for amalfi that I've seen before include "Italian resort on the Gulf of Sorrento" , "Italian holiday coast" , "Italian location" , "Resort in South Italy" , "Italian town on Gulf of Salerno" .)