2a. Wastrel less in the red? (5) B?U?R the answer
is probably BLUER,but I have no idea why.The same
goes for 26a.Backing Dadaist to make exhibits
here? (5) P?A?O The answer is PRADO,why?
I think Bluer comes from the fact that the other end of the spectrum to red is blue and bluer sounds like blewer as in spending all ones money.
As for Prado I don't know, the web just tells us it is a really really famous gallery/museum. One of those answers that come from experience rather than outright logic!