Soldiers without skill lacking in force (4)
I believe the answer is:
unit
'soldiers' is the definition.
(unit can mean a group of soldiers )
'without skill lacking in force' is the wordplay.
'without skill' becomes 'unfit' (not fit for a job).
'lacking in' indicates named letters should be taken away.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'unfit' with 'f' taken away is 'UNIT'.
(Other definitions for unit that I've seen before include "Subdivision of a whole" , "Element or part" , "Single item; one" , "Individual component" , "Mexican maize pancake" .)