Soldiers must surround a base (4)
I believe the answer is:
mean
'base' is the definition.
('mean' can be a synonym of 'base')
'soldiers must surround a' is the wordplay.
'soldiers' becomes 'men' (I've seen this before).
'must surround' means one lot of letters goes inside another (I've seen this in other clues).
'men' enclosing 'a' is 'MEAN'.
(Other definitions for mean that I've seen before include "spell" , "Petty" , "'Midway, average (4)'" , "Have in mind to be miserly?" , "Ungenerous" .)