Composing trio is a lot of fun (4)
I believe the answer is:
riot
'lot of fun' is the definition.
'composing trio' is the wordplay.
'composing' is an anagram indicator.
'trio' is an anagram of 'RIOT'.
'is a' is the link.
(Other definitions for riot that I've seen before include "A public disturbance by crowd" , "Dazzling display - revelry" , "A wild public affray" , "Lavish display (of colour)" , "loose living" .)