Shareholder's substitute (5)
I believe the answer is:
proxy
'shareholder's substitute' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for proxy that I've seen before include "Agent (for eg voting)" , "Person voting on behalf of another" , "One voting on behalf of another" , "Person authorised to act on behalf of another" , "Authorised agent" .)