Aussie native going over top of Kilkenny Castle? (4)
I believe the answer is:
rook
'castle?' is the definition.
('rook' can be a synonym of 'castle')
'aussie native going over top of kilkenny' is the wordplay.
'aussie native' becomes 'roo' (I've seen this in other clues).
'going over' is a charade indicator (letters next to each other).
'top of' says to take the initial letters.
The initial letter of 'kilkenny' is 'k'.
'roo'+'k'='ROOK'
(Other definitions for rook that I've seen before include "Another name for castle in chess" , "Rank - and - file man" , "Fleece - bird" , "journalist Jean" , "Swindle (colloq.); bird" .)