Representative for unknown figures (5)
I believe the answer is:
proxy
'representative' is the definition.
(I know that proxy is a type of agent)
'for unknown figures' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'unknown figures' becomes 'xy' (algebra).
'pro'+'xy'='PROXY'
(Other definitions for proxy that I've seen before include "Person acting for another" , "Authorised substitution" , "Authorised substitute" , "Person voting on behalf of another" , "Delegated vote" .)