Infantry base (4)
I believe the answer is:
foot
I believe this clue is a double definition.
'infantry' is the first definition.
(foot soldiers)
'base' is the second definition.
(I know that base can be written as foot)
(Other definitions for foot that I've seen before include "Distance to the end of a limb?" , "See 24" , "Metrical unit; pay (a bill)" , "Imperial distance" , "An imperial length or body part" .)