Ordinary sort of castle maybe for a knight to occupy (7)
I believe the answer is:
vanilla
'ordinary' is the definition.
(I've seen this before)
'for a knight to occupy' is the wordplay.
'for' becomes 'villa' (I am not sure about this - if you are sure you should believe this answer much more).
'knight' becomes 'n' (chess).
'to occupy' is an insertion indicator.
'a'+'n'='an'
'villa' enclosing 'an' is 'VANILLA'.
'sort of castle maybe' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for vanilla that I've seen before include "Pod used in cookery" , "Plain" , "Subtle flavour pod" , "Common ice cream flavour" , "Popular ice cream flavour" .)