Kelly Clarkson isn’t holding back -- firing off at her ex-manager who once suggested she get a boob job ... and her response back to him definitely hit all the right notes!

The singer was mid-show at her "Studio Sessions" at Caesars Palace in Las Vegas, Saturday night when her monologue turned to her cleavage ... after joking she was spilling out of her low-cut look, Kelly reflected, "I had a d*ck manager once tell me to get a boob job!"

Kelly was totally floored by the so-called "advice" ... telling him she suggested he get a 'd*ck job" instead.

She doubled down, saying she’s always been perfectly happy with her "itty-bitty titties" ... and reminding fans that, yes, people in the industry really do say wild stuff like that all the time. Gallery Launch Gallery Getty

Kelly said

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