The ultimate source of, say, port's rumour? (9)
I believe the answer is:
grapevine
'rumour?' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for grapevine that I've seen before include "(Informal) means of spreading information?" , "Fruit-bearing plant - rumour mill" , "Informal source of information" , "Bush telegraph" , "fruit producer" .)