Join masses dancing for music performance (3,7)
I believe the answer is:
jam session
'music performance' is the definition.
(improvised musical performance)
'join masses dancing' is the wordplay.
'dancing' is an anagram indicator.
'join'+'masses'='joinmasses'
'joinmasses' is an anagram of 'JAM SESSION'.
'for' acts as a link.
(Other definitions for jam session that I've seen before include "improvised performance" , "jazz performance" , "time for a sarnie?" , "Impromptu 22 concert" , "Gathering of jazz musicians" .)