Bomb was humming — second to be dropped (4)
I believe the answer is:
tank
'bomb' is the definition.
(both can mean to fail)
'was humming second to be dropped' is the wordplay.
'was humming' becomes 'stank' (hum can mean to stink).
'second' becomes 's' (common abbreviation).
'to be dropped' indicates named letters should be taken away.
'stank' with 's' taken out is 'TANK'.
(Other definitions for tank that I've seen before include "Miltary vehicle" , "defeat" , "Possible alternative to barrel" , "Offensive vehicle" , "Vat" .)