Pay for infantry (4)
I believe the answer is:
foot
This clue is a double definition.
'pay for' is the first definition.
(as in to foot the bill)
'infantry' is the second definition.
(foot soldiers)
(Other definitions for foot that I've seen before include "Measure - part of body" , "An imperial length or body part" , "Imperial distance" , "Distance to the end of a limb?" , "Labour Party leader, 1980-83" .)