Instrument placed on nitrogen bomb (4)
I believe the answer is:
nuke
'bomb' is the definition.
(nuking is a kind of bombing)
'instrument placed on nitrogen' is the wordplay.
'instrument' becomes 'uke' (uke is a kind of instrument).
'placed on' says to put letters next to each other.
'nitrogen' becomes 'n' (N is the chemical symbol for nitrogen).
'uke' put after 'n' is 'NUKE'.
(Other definitions for nuke that I've seen before include "Attack with an atomic weapon" , "Attack with H-bomb (slang abbr.)" , "Nuclear weapon (sl.)" , "Destroy with a very powerful 18" , "Drop huge bomb on" .)