Cause or catalyst (5)
I believe the answer is:
agent
I believe this clue is a double definition.
'cause' is the first definition.
(agent is a kind of cause)
'catalyst' is the second definition.
(I've seen this before)
(Other definitions for agent that I've seen before include "Bond, e.g." , "One does business on behalf of another" , "that may be secret?" , "One acting on behalf of another" , "Double-zero-seven, for example" .)