3d. I in PLANS + AILING
1d. P + OPE (short form of open, used poetically)
16a. NEWT (as in, as drunk as a …) + ON (regarding)
5d. A + UNTIE (free)
20a. TEMP (acting role) + LATE (former)
Re 5a....more finger trouble from me. Not only did I post in the wrong thread, my phone autocorrected DUCHESS to DUCHESSES. So you're correct, it should be singular.