Brothers over six feet? Too much to ask (4,5)
I believe the answer is:
tall order
'brothers over six feet? too much' is the definition.
I can't judge whether this definition defines the answer.
'to ask' is the wordplay.
'to' becomes 'tall' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ask' becomes 'order' (ordering is a kind of asking).
'tall'+'order'='TALL ORDER'
Can you help me to learn more?
(Other definitions for tall order that I've seen before include "delivery of sequoias?" , "Lot to ask" , "demanding request" , "for brotherhood of high standing" , "Lofty command - that's a difficult task" .)