The preserve of improvising musicians? (3,7)
I believe the answer is:
jam session
'the preserve of improvising musicians?' is the definition.
I can't judge whether this defines the answer.
'the preserve of improvising' is the wordplay.
'the preserve' becomes 'jam' (jam is a kind of preserve).
'of improvising' becomes 'session' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'jam'+'session'='JAM SESSION'
Can you help me to learn more?
(Other definitions for jam session that I've seen before include "improvised performance" , "jazz performance" , "time for a sarnie?" , "playing of improvised bebop?" , "Gathering of jazz musicians" .)