Too much to ask, request for large coffee? (4,5)
I believe the answer is:
tall order
'too much to ask' is the definition.
(a tall order is a demanding request)
'request for large coffee?' is the wordplay.
I cannot really understand how this works, but
'request' could be 'order' (an order is a request from someone in authority) and 'order' is present in the answer.
'large' could be 'tall' (both can mean big) and 'tall' is present in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for tall order that I've seen before include "Lofty command - that's a difficult task" , "Difficult assignment" , "Difficult demand" , "Formidable task" , "as the building of Babel was?" .)