Soldiers retaining force would be inappropriate (5)
I believe the answer is:
unfit
'inappropriate' is the definition.
(I've seen this before)
'soldiers retaining force' is the wordplay.
'soldiers' becomes 'unit' (unit can mean a group of soldiers ).
'retaining' is an insertion indicator.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'unit' placed around 'f' is 'UNFIT'.
'would be' is the link.
(Other definitions for unfit that I've seen before include "Not in good shape physically" , "Ill-equipped" , "Not prepared" , "Not in good physical condition, or not suitable" , "Incapable" .)