Cardinal, fine, welcoming Rome’s capital support of course (8)
I believe the answer is:
thirteen
'cardinal' is the definition.
('thirteen' can be similar in meaning to 'cardinal')
'fine welcoming rome's capital support of course' is the wordplay.
'fine' becomes 'thin' ('thin' can be similar in meaning to 'fine'**).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'capital' suggests taking the first letters.
'support of course' becomes 'tee' (support for ball on golf course).
The initial letter of 'romes' is 'r'.
'r'+'tee'='rtee'
'thin' placed around 'rtee' is 'THIRTEEN'.
(Other definitions for thirteen that I've seen before include "Number considered unlucky" , "Baker's dozen" , "Supper attendees" , "Where situation is" , "Cardinal" .)