Assault force becoming excited (3,2,4)
I believe the answer is:
set on edge
'excited' is the definition.
(as in setting teeth on edge)
'assault force' is the wordplay.
'assault' becomes 'set on' (synonyms).
'force' becomes 'edge' (I've seen this before).
'seton'+'edge'='SET ON EDGE'
'becoming' is the link.
(Other definitions for set on edge that I've seen before include "stirred" , "Excite" .)