Retain stronghold (4)
I believe the answer is:
keep
I believe this clue is a double definition.
'retain' is the first definition.
('keep' can be a synonym of 'retain')
'stronghold' is the second definition.
(keep is a kind of stronghold)
(Other definitions for keep that I've seen before include "Castle tower; cost of maintenance" , "Only one cricketer on field will" , "Conserve" , "Eg, beef" , "Retain; part of castle" .)