Remain in a fortified place (4)

I believe the answer is:

keep

I'm a little stuck... Click here to teach me more about this clue!

'remain' is the definition.
(I know that keep is a more specific form of the action remain)

I don't understand how the rest of the clue works.

(Other definitions for keep that I've seen before include "run" , "Retain; strong tower" , "Guard" , "Fort" , "Retain or care for" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app