Opening for hearing next to old currency reserve (7)
I believe the answer is:
earmark
'reserve' is the definition.
(I know that reserve can be written as earmark)
'opening for hearing next to old currency' is the wordplay.
'opening for hearing' becomes 'ear' (opening in the body).
'next to' says to put letters next to each other.
'old currency' becomes 'mark' (I've seen this in other clues).
'ear'+'mark'='EARMARK'
(Other definitions for earmark that I've seen before include "'Reserve, set aside (7)'" , "Set aside for special purpose" , "Set aside specifically" , "It identifies the outcome" , "Reserve specially" .)