Acquired old rubbish that will probably not be used (2,2,5)
I believe the answer is:
go to waste
'will probably not be used' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'acquired old rubbish' is the wordplay.
I cannot really understand how this works, but
'acquired' could be 'got' ('get' can be a synonym of 'acquire') and 'got' is found within the answer.
'old' could be 'o' (abbreviation) and 'o' is found within the answer.
'rubbish' could be 'waste' (I've seen this before) and 'waste' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'that' acts as a link.
Can you help me to learn more?
(Other definitions for go to waste that I've seen before include "Be misused" , "Not get used profitably" .)