Bound to be fired after penning note (5)
I believe the answer is:
limit
'bound' is the definition.
(limit is a kind of bound)
'fired after penning note' is the wordplay.
'fired' becomes 'lit' (set on fire).
'after penning' is an insertion indicator (pen can mean to imprison or contain).
'note' becomes 'mi' (as in 'do re mi').
'lit' enclosing 'mi' is 'LIMIT'.
'to be' acts as a link.
(Other definitions for limit that I've seen before include "Curtail" , "Restrict - boundary" , "Cutoff point" , "Confine to specific boundary" , "control" .)