Organ gang is burning (7)
I believe the answer is:
earnest
'is burning' is the definition.
(thesaurus)
'organ gang' is the wordplay.
'organ' becomes 'ear' (ear is a kind of organ).
'gang' becomes 'nest' (nest is a kind of gang**).
'ear'+'nest'='EARNEST'
(Other definitions for earnest that I've seen before include "Sincere -- humourless" , "Sincere or over-serious" , "Solemn; pledge" , "Determined - heartfelt" , "money put down" .)