21a Hard plays periodically put on under canvas (8) I-T-N---
Looks like INTENTLY but I can't completely parse it.
ut I can
19d Plenty whine after going outside (8) O----N-E
Looks like OPULENCE but I can't completely parse this either.
18d Surmounting obstacle is skinny runner's challenge (8) G----L-T
Looks like GAUNTLET, but I don't understand why LET = obstacle...unless as in tennis?