Mostly secretive pork-pie manufacturer, one supplying Italian capital (8)
I believe the answer is:
cagliari
'italian capital' is the definition.
(capital of Sardinia)
'mostly secretive pork-pie manufacturer one' is the wordplay.
'mostly' means to remove the last letter.
'secretive' becomes 'cagy' (I've seen this before).
'pork-pie manufacturer' becomes 'liar' ('pork pie' is Cockney rhyming slang for 'lie').
'one' becomes 'i' (Roman numeral).
'cagy' with its last letter taken away is 'cag'.
'cag'+'liar'+'i'='CAGLIARI'
'supplying' is the link.
(Other definitions for cagliari that I've seen before include "Sardinian capital" , "Capital of Sardinia" .)