Ordinary people that are controlled by castle (4,3,4)
I believe the answer is:
rank and file
'ordinary people' is the definition.
(as in rank and file members)
'are controlled by castle' is the wordplay.
I cannot quite understand how this works, but
'are' could be 'ran' (running is a kind of being) and 'ran' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'that' acts as a link.
(Other definitions for rank and file that I've seen before include "controlled by the castle" , "Ordinary people as opposed to their leaders" , "Ordinary men" , "rows?" , "seen on a chessboard" .)