Pop turned back as I entered to say goodbye (5)
I believe the answer is:
adios
'say goodbye' is the definition.
(I know that adios is a class)
'pop turned back as i' is the wordplay.
'pop' becomes 'da' (both can mean someone's father).
'turned' is a reversal indicator.
'back' is a reversal indicator.
'as' becomes 'so' (I've seen this in other clues).
'da' backwards is 'ad'.
'so'+'i'='soi'
'soi' in reverse letter order is 'ios'.
'ad'+'ios'='ADIOS'
'entered to' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for adios that I've seen before include "Au revoir, amigo!" , "Goodbye, senor" , "See you in Ibiza" , "Bye!" , "Farewell amigo!" .)