I’m one having volunteers to run copier (8)
I believe the answer is:
imitator
'copier' is the definition.
(thesaurus)
'i'm one having volunteers to run' is the wordplay.
'one' becomes 'i' (Roman numeral).
'having' is a charade indicator (letters next to each other).
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'run' becomes 'r' (cricket abbreviation).
'im'+'i'+'ta'+'to'+'r'='IMITATOR'
(Other definitions for imitator that I've seen before include "One who mimics or emulates" , "Someone copying the behaviour of another" , "follower" , "A trio, Tim, is a mimic" , "Ape" .)