Pen with no top hampered competitor (8)
I believe the answer is:
opponent
'competitor' is the definition.
(opponent is a kind of competitor)
'pen with no top hampered' is the wordplay.
'with' says to put letters next to each other.
'hampered' is an anagram indicator ('hampered' can mean entangled).
'pen'+'no'+'top'='pennotop'
'pennotop' is an anagram of 'OPPONENT'.
(Other definitions for opponent that I've seen before include "Adversary, competitor" , "Rival" , "'Adversary, antagonist (8)'" , "Foe or competitor" , "The one you're up against" .)