Slackening rhymes with:
blackening, blackening, awakening, beckoning, blackening, chickening, darkening, dekoning, likening, quickening, reawakening, reckoning, sickening, thickening, weakening, wilkening
Slackening sounds like:
salesman's, salesmen's, silicones, siliconix, slackens, slackness, slashing, slicing, slocan's, slogans, slogging, sloshing, slouching, slugging, sluggishness, sluicing, socializing, squelching, sulking