Am really stuck on last few:
15a overlook writings about one of the Romans [4]: -I-S
17a sign that's oddly place in airliners [5]: A-I-S
20a on which gymnasts may perform as sign of equality, perhaps [10,4]: H-R---N-A- BARS
21d what's repeated in jazz variation for their instrument [6]: --t-e-
These all link in and I think I may have gone wrong somewhere!