Rivals put top on pens wrongly (9)
I believe the answer is:
opponents
'rivals' is the definition.
(opponent is a kind of rival)
'top on pens wrongly' is the wordplay.
'wrongly' is an anagram indicator.
'top'+'on'+'pens'='toponpens'
'toponpens' is an anagram of 'OPPONENTS'.
'put' acts as a link.
(Other definitions for opponents that I've seen before include "matches" , "The other side" .)