Sentence:  But the night's true indictment of country's man problem was the dullness of the performances by male stars exponentially more famous than Mr. Stapleton: Zac Brown, <WSD>undersinging</WSD> on “Beautiful Drug”; Kenny Chesney's lethargic turn on “Save It for a Rainy Day”; Blake Shelton mugging for the cameras on “Gonna,” Dierks Bentley needlessly stoic on “Riser.” 
Meaning of the word marked with <WSD> tags:  To sing inadequately, or with too little vocal effort.