Remember when Le'Veon Bell posted one of his new rap vids, asked people what they thought about it ... and Keenan Allen hit him back with the brutal "this ain't it" response??
Well ... here's Bell's chance to get revenge ... 'cause the L.A. Chargers star posted a video of himself dabbling into music -- singin' a Daniel Caesar song and playing the piano!!
The initial exchange between the 2 NFLers got personal -- with Bell calling Allen a troll, and KA claiming he was just keeping it real.