A mere error, the agent holds, in the contract (9)
I believe the answer is:
agreement
'the contract' is the definition.
(thesaurus)
'a mere error the agent holds' is the wordplay.
'error' indicates an anagram.
'holds' means one lot of letters goes inside another.
'mere' is an anagram of 'reem'.
'reem' placed into 'agent' is 'AGREEMENT'.
'in' acts as a link.
(Other definitions for agreement that I've seen before include "Harmony, concurrence" , "Accord, contract" , "Pact" , "Contract or promise" , "Harmony of opinion" .)