Drilled military men occupying base (5)
I believe the answer is:
bored
'drilled' is the definition.
('bore' can be a synonym of 'drill')
'military men occupying base' is the wordplay.
'military men' becomes 'OR' (Other Ranks - low-ranking soldiers).
'occupying' is an insertion indicator (inserted letters occupy or enter the word).
'base' becomes 'bed' (both can mean the bottom of something).
'or' placed within 'bed' is 'BORED'.
(Other definitions for bored that I've seen before include "Extremely uninterested" , "Drilled; fed up" , "Weary; drilled" , "Short of things to do, maybe" , "Filled with tedium" .)