After playing both sides of cassette (4)
I believe the answer is:
once
'after' is the definition.
(once can mean immediately after)
'playing both sides of cassette' is the wordplay.
'playing' becomes 'on'.
'both sides of' says to hollow out the word (remove centre letters) (only the letters on the sides remain).
'cassette' with its middle removed is 'ce'.
'on'+'ce'='ONCE'
(Other definitions for once that I've seen before include "Formerly - not repeated" , "Singly" , "in days of yore?" , "yesterday" , "At some former time" .)