Harry Potter cast shortly watching one (9)
I believe the answer is:
spectator
'watching one' is the definition.
(a person watching something)
'harry potter cast shortly' is the wordplay.
'harry' indicates an anagram (harry can mean to attack or spoil).
'shortly' means to remove the last letter.
'potter'+'cast'='pottercast'
'pottercast' with its last letter removed is 'pottercas'.
'pottercas' anagrammed gives 'SPECTATOR'.
(Other definitions for spectator that I've seen before include "Part of gate" , "member of the crowd" , "He attends matches" , "fan" , "Onlooker at game" .)