Praise old key (5)
I believe the answer is:
exalt
'praise' is the definition.
(I've seen this before)
'old key' is the wordplay.
'old' becomes 'ex'.
'key' becomes 'alt' (I've seen this in other clues).
'ex'+'alt'='EXALT'
(Other definitions for exalt that I've seen before include "Elevate" , "Raise high, glorify" , "(P) raise" , "delight" , "Extol" .)