Actual rock for agent (7)
I believe the answer is:
realtor
'agent' is the definition.
(realtor is a kind of agent)
'actual rock' is the wordplay.
'actual' becomes 'real' (similar in meaning).
'rock' becomes 'tor' (tor is a kind of rock).
'real'+'tor'='REALTOR'
'for' acts as a link.
(Other definitions for realtor that I've seen before include "One sells properties" , "American estate agent" , "US estate agent" , "Estate agent in America" .)