Firm's paid regularly to supply fighters (4)
I believe the answer is:
impi
'fighters' is the definition.
(I've seen this before)
'firm's paid regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'firms'+'paid'='firmspaid'
The alternating letters of 'firmspaid' are 'IMPI'.
'to supply' acts as a link.
(Other definitions for impi that I've seen before include "Body of Zulu warriors" , "Fighting force" , "group of fighting men" , "African warriors" , "Zulu warrior tribe" .)