Retain the stronghold (4)
I believe the answer is:
keep
This clue is a double definition.
'retain' is the first definition.
(I know this)
'stronghold' is the second definition.
(I know that keep is a type of stronghold)
'the' is the link.
(Other definitions for keep that I've seen before include "Retain possession of" , "Continue" , "Remain" , "Guard" , "Hold - part of castle" .)