Assistant's private call about becoming associate (9)
I believe the answer is:
secretary
'assistant's' is the definition.
(secretary is a kind of assistant)
'private call about becoming associate' is the wordplay.
I cannot quite see how this works, but
'private' could be 'secret' (similar in meaning) and 'secret' is found within the answer.
'about' could be 'a' (abbreviation in Chambers) and 'a' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for secretary that I've seen before include "Assistant who handles correspondence" , "Personal assistant" , "minister's assistant" , "Ambassador's assistant" , "PA" .)