Fidelio's Léonore, originally in disguise here: well, well, well! (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Anagram of FIDELIO’S + L[éonore]. Leonore does indeed disguise herself as Fidelio at the start of Beethoven’s opera: I’m not sure why Paul has used the French form of her name here