Design of base stolen by soldier (4)
I believe the answer is:
mean
'design' is the definition.
(I know that design is a more specific form of the action mean)
'base stolen by soldier' is the wordplay.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'stolen by' is an insertion indicator (inserted letters are taken or stolen).
'soldier' becomes 'man' (man can mean a soldier).
'e' going inside 'man' is 'MEAN'.
'of' acts as a link.
(Other definitions for mean that I've seen before include "Intend - shabby" , "cause" , "very good" , "Norm" , "middle course" .)