Rival pages one getting in on time (8)
I believe the answer is:
opponent
'rival' is the definition.
(a rival is an opponent or enemy)
'pages one getting in on time' is the wordplay.
'pages' becomes 'pp' (an abbreviation for page written twice).
'getting in' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'pp'+'one'='ppone'
'ppone' going into 'on' is 'opponen'.
'opponen'+'t'='OPPONENT'
(Other definitions for opponent that I've seen before include "Challenger" , "Contestant you're matched against" , "'Adversary, antagonist (8)'" , "Foe or competitor" , "Adversary, competitor" .)