Retain possession of stronghold (4)
I believe the answer is:
keep
This clue is a double definition.
'retain possession of' is the first definition.
(I've seen this before)
'stronghold' is the second definition.
(keep is a kind of stronghold)
(Other definitions for keep that I've seen before include "Conserve" , "Central tower of castle" , "Main tower of a medieval castle" , "Save" , "Only one cricketer on field will" .)