Penetrating entrance at the back of empty tower (9)
I believe the answer is:
trenchant
'penetrating' is the definition.
(I've seen this in another clue)
'entrance at the back of empty tower' is the wordplay.
'entrance' becomes 'enchant' (to entrance is to enchant or bewitch).
'at the back of' says to put letters next to each other.
'empty' means to remove the middle letters.
'tower' with its centre removed is 'tr'.
'enchant' after 'tr' is 'TRENCHANT'.
(Other definitions for trenchant that I've seen before include "Acerbic" , "Keen" , "Forthright, vigorous" , "Clear" , "Expressively incisive" .)