Republican centre supporter brought up nonsense (7)
I believe the answer is:
rhubarb
'nonsense' is the definition.
(I've seen this before)
'republican centre supporter brought up' is the wordplay.
'republican' becomes 'r' (abbreviation).
'centre' becomes 'hub' (hub is a kind of centre**).
'supporter' becomes 'bra' (I've seen this before).
'brought up' says the letters should be written in reverse (down clues only).
'bra' written backwards gives 'arb'.
'r'+'hub'+'arb'='RHUBARB'
(Other definitions for rhubarb that I've seen before include "Indistinct stage chattering noise by actors" , "Stalky fruit" , "Garden plants with fleshy stalks or slang nonsense" , "Noise of actors simulating conversation" , "Fruit - nonsense!" .)