Substitute with mission to keep record (6)
I believe the answer is:
deputy
'substitute' is the definition.
(both can mean someone who stands in for another)
'mission to keep record' is the wordplay.
'mission' becomes 'duty' (someone's mission is their duty or task).
'to keep' is an insertion indicator.
'record' becomes 'ep' (abbreviation for extended play record).
'duty' placed around 'ep' is 'DEPUTY'.
'with' is the link.
(Other definitions for deputy that I've seen before include "Subordinate" , "Assistant with power to act in the absence of superior" , "Person appointed to substitute for another" , "Second in command" , "Person standing in" .)