This castle is on a sort of rock (6)
I believe the answer is:
glamis
'this castle' is the definition.
(I've seen this before)
'is on a sort of rock' is the wordplay.
'on' says to put letters next to each other.
'a sort of rock' becomes 'glam' (I've seen this before).
'is' after 'glam' is 'GLAMIS'.
(Other definitions for glamis that I've seen before include "Village in Angus with castle where the Queen's mother grew up" , "Home of Macbeth and the Queen" , "Scottish castle, once Macbeth's" , "Angus castle, legendary Monster" .)