Swap notes during drill (5-2)
I believe the answer is:
trade-in
'swap' is the definition.
(thesaurus)
'notes during drill' is the wordplay.
'notes' becomes 'de' (two musical notes).
'during' means one lot of letters goes inside another.
'drill' becomes 'train' (I've seen this in another clue).
'de' put into 'train' is 'TRADE-IN'.
(Other definitions for trade-in that I've seen before include "I daren't (anag)" , "Article used in part payment for another" , "Older article used in part payment to retailer" , "Something used in part exchange" , "Old item given in part payment for a new one" .)