Potentially never-ending series then, each set of years (4,5)
I believe the answer is:
soap opera
'potentially never-ending series' is the definition.
'soap opera' can be an answer for 'series' (soap opera is a kind of series). I am unsure of the 'potentially never-ending' bit.
'then each set of years' is the wordplay.
I cannot really understand how this works, but
'then' could be 'so' (both can mean 'therefore' or 'in that case') and 'so' is found in the answer.
'each' could be 'apop' (eg 'one pound a pop' can mean 'one pound each') and 'apop' is present in the answer.
'years' could be 'era' (I've seen this in another clue) and 'era' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for soap opera that I've seen before include "TV show" , "Television serial" , "Sentimental TV serial, sounds clean" , "Serialised programme" , "Fair City or Coronation Street, say" .)