Record number (5)
'record number' is the definition.
(song on a vinyl record)
'record number' is the wordplay.
'record' becomes 'trac' (I can't justify this - if you can you should believe this answer much more).
'number' becomes 'K' (K is an example).
'trac'+'k'='TRACK'
Can you help me to learn more?
(Other definitions for track that I've seen before include "Follow < course" , "One of the songs on CD?" , "racecourse" , "Mark - line - groove" , "One song on CD?" .)