1a Dismally - 'small' (little, in) 'DIY'
2d Strangler - 'angle' (fish, in) 'str(ait) + r(iver)'
6d Managua - 'man' + 'agua' (Sp. "water"; main; sea)
16d (p)Ortolan (book of routes, uncovered)
24a Unplug - 'gulp (noisily swallow) n(ew) u(pper-class)' (round; back)