Discharged incendiary bomb in action (8)
I believe the answer is:
'discharged' is the definition.
(demobbing is a kind of discharging)
'incendiary bomb in action' is the wordplay.
I cannot really understand how this works, but
an anagram of 'bomb' is 'mobb' which is present in the answer.
'action' could be 'deed' (deed is a kind of action) and 'deed' is found in the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for demobbed that I've seen before include "Returned to civvy street" , "discharged" , "Retired from military service" .)