Spiteful soldiers seizing leader in army (4)
I believe the answer is:
'spiteful' is the definition.
'soldiers seizing leader in army' is the wordplay.
'soldiers' becomes 'men' (men can mean a group of soldiers).
'seizing' means one lot of letters goes inside another (inserted letters are seized).
'leader in' says to take the initial letters.
The first letter of 'army' is 'a'.
'men' placed around 'a' is 'MEAN'.
(Other definitions for mean that I've seen before include "Name for being tight-fisted" , "cause" , "Base; tight" , "Signify; average" , "design" .)