War vehicle's function: kill, primarily (4)
I believe the answer is:
tank
'war vehicle's' is the definition.
(I've seen this in another clue)
'function kill primarily' is the wordplay.
'function' becomes 'tan' (tangent, mathematical function).
'primarily' suggests taking the first letters.
The initial letter of 'kill' is 'k'.
'tan'+'k'='TANK'
(Other definitions for tank that I've seen before include "Large storage chamber" , "Aquarium; army vehicle" , "Container for liquids - military vehicle" , "War machine" , "A military vehicle or large container" .)