Combat vehicle's brown and khaki, primarily (4)
I believe the answer is:
tank
'combat vehicle's' is the definition.
(I've seen this in another clue)
'brown and khaki primarily' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'and' means one lot of letters go next to another.
'primarily' suggests taking the first letters.
The first letter of 'khaki' is 'k'.
'tan'+'k'='TANK'
(Other definitions for tank that I've seen before include "Offensive vehicle" , "Container for liquids - military vehicle" , "Large storage chamber" , "Septic container" , "Armoured vehicle - container for liquid or gas" .)