About how old the façade is, be of one mind (5)
I believe the answer is:
agree
'be of one mind' is the definition.
(I've seen this before)
'about how old the facade' is the wordplay.
'about' indicates putting letters inside.
'how old' becomes 'age' (I've seen this before).
'the facade' becomes 're' (I can't explain this - if you can you should believe this answer much more).
'age' placed around 're' is 'AGREE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for agree that I've seen before include "concede" , "Concur, coincide" , "Hold the same view" , "'Birds in their little nest . . . . .'" , "suit" .)