Some person a mediator called (5)
I believe the answer is:
named
'called' is the definition.
(synonyms)
'some person a mediator' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'NAMED' can be found hidden inside 'person a mediator'.
(Other definitions for named that I've seen before include "Christened" , "Handled" , "Designated" , "Entitled" , "Identified by appellation" .)