Probably "grenade". A flashbang grenade is a bright and loud but less destructive grenade used to startle people before an attack. Widely used by American police investigating possession of trivial amounts of marijuana.
In that game, the number said after the clue tells you how many of the words in play are related to the clue. So the word flashbang wouldn't be hinting at a codeword, but four of them.
So it could really be anything, e.g. bang, army, light and blind.
Since it's codenames, I would guess some of the words on the board they were hinting at would be WW2/war/call of duty/video games related stuff like France, Ear, Lag, Light, Hand, Hurt, Mud, Radio...
Something police use to disorient and theoretically help disperse a crowd - like a grenade except it creates only a mostly-harmless noise and flash of light.
Not that harmless. They don't throw fragmentation shrapnel but they are strong enough to punch holes into people when in direct vicinity. US police regularly throw these into cribs and kill or maim babies and toddlers.
