Soldiers surrounding a base (4)
I believe the answer is:
mean
'base' is the definition.
(synonyms)
'soldiers surrounding a' is the wordplay.
'soldiers' becomes 'men' (I've seen this before).
'surrounding' is an insertion indicator.
'men' enclosing 'a' is 'MEAN'.
(Other definitions for mean that I've seen before include "'Midway, average (4)'" , "Signify; miserly" , "Intend - shabby" , "Bad-tempered" , "Type of average" .)