Piano to discard? That causes huge depression (5,6)
I believe the answer is:
grand canyon
'huge depression' is the definition.
The definition suggests a singular noun which matches the answer.
'piano to discard? that' is the wordplay.
'piano' becomes 'grand piano' (grand piano is a kind of piano).
'to discard?' becomes 'can' (I can't justify this - if you can you should give a lot more credence to this answer).
'that' becomes 'yon' (archaic term meaning 'that').
'grand'+'can'+'yon'='GRAND CANYON'
'causes' acts as a link.
Can you help me to learn more?
(Other definitions for grand canyon that I've seen before include "Colorado River gorge" , "Arizona tourist attraction" , "nickname of US State" , "Major feature in Arizona" , "American gorge" .)