Pop-up books about United's rival (8)
I believe the answer is:
opponent
'rival' is the definition.
(a rival is an opponent or enemy)
'pop-up books about united's' is the wordplay.
'pop-up' becomes 'opp' (I can't justify this - if you can you should give a lot more credence to this answer).
'books' becomes 'NT' (New Testament).
'about' is a charade indicator (letters next to each other).
'united' becomes 'one' (as in a phrase like 'we are one').
'nt' put after 'one' is 'onent'.
'opp'+'onent'='OPPONENT'
Can you help me to learn more?
(Other definitions for opponent that I've seen before include "member of other team" , "'Adversary, antagonist (8)'" , "One's competitor" , "The one you're up against" , "Foe or competitor" .)