A group of musicians playing, showing lack of restraint (7)
I believe the answer is:
abandon
'lack of restraint' is the definition.
(I've seen this before)
'a group of musicians playing' is the wordplay.
'group of musicians' becomes 'band' (I've seen this before).
'playing' becomes 'on'.
'a'+'band'+'on'='ABANDON'
'showing' acts as a link.
(Other definitions for abandon that I've seen before include "Give up (something) completely" , "Leave, desert" , "Wantonness: desert" , "Leave high and dry" , "Forsake - careless freedom" .)