One following proposer once Reds are involved (8)
I believe the answer is:
seconder
'one following proposer' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'once reds are involved' is the wordplay.
'are involved' indicates an anagram (involved can mean confusing or complex).
'once'+'reds'='oncereds'
'oncereds' with letters rearranged gives 'SECONDER'.
Can you help me to learn more?
(Other definitions for seconder that I've seen before include "Person supporting" , "One formally supporting" , "Supporter of a motion" .)