One representing another sister ringing old employer, perhaps (5,4)
I believe the answer is:
agent noun
'employer perhaps' is the definition.
('employer' is an example of an '-er' agent noun)
'one representing another sister ringing old' is the wordplay.
'one representing another' becomes 'agent'.
'sister' becomes 'nun' (I've seen this before).
'ringing' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'nun' placed around 'o' is 'noun'.
'agent'+'noun'='AGENT NOUN'
(Other definitions for agent noun that I've seen before include "lecturer, for instance" , "leader, perhaps" , "lecturer, say" , "Writer is one" .)