Magician's striking entrance holding top hat (9)
I believe the answer is:
enchanter
'magician's' is the definition.
(I've seen this before)
'striking entrance holding top hat' is the wordplay.
'striking' indicates anagramming the letters ('strike' can mean 'hit' or 'disrupt').
'holding' means one lot of letters goes inside another.
'top' indicates taking the first letters.
The first letter of 'hat' is 'h'.
'entrance' is an anagram of 'encanter'.
'encanter' going around 'h' is 'ENCHANTER'.
(Other definitions for enchanter that I've seen before include "magician" , "siren, possibly" , "One charming" .)