Soldiers circling a base (4)
I believe the answer is:
mean
'base' is the definition.
(I know that base can be written as mean)
'soldiers circling a' is the wordplay.
'soldiers' becomes 'men' (men can mean a group of soldiers).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'men' placed around 'a' is 'MEAN'.
(Other definitions for mean that I've seen before include "Petty" , "Being near" , "Name for being stingy" , "middle course" , "mercenary" .)