Gathering one street is red (9)
I believe the answer is:
socialist
'red' is the definition.
(I know that red can be written as socialist)
'gathering one street' is the wordplay.
'gathering' becomes 'social' (social is a kind of gathering).
'one' becomes 'i' (Roman numeral).
'street' becomes 'st' (abbreviation).
'social'+'i'+'st'='SOCIALIST'
'is' acts as a link.
(Other definitions for socialist that I've seen before include "Political theorist" , "Fabian" , "See 1" , "Politician of the left" , "Lefty" .)