Dirt making uniforms dirty, primarily (3)
I believe the answer is:
mud
'dirt' is the definition.
(mud is a kind of dirt)
'making uniforms dirty primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'making uniforms dirty' is 'MUD'.
(Other definitions for mud that I've seen before include "slander" , "Muck - actionable remarks" , "in your eye, it's an affable toast" , "Here's . . . in your eye - it's a toast" , "-, -, Glorious - (Flanders and Swann)" .)