Force that is keeping military police liberal (5)
I believe the answer is:
impel
'force' is the definition.
(I know that force can be written as impel)
'that is keeping military police liberal' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'keeping' indicates putting letters inside (some letters kept inside).
'military police' becomes 'MP' (abbreviation).
'liberal' becomes 'l' (abbreviation).
'ie' going around 'mp' is 'impe'.
'impe'+'l'='IMPEL'
(Other definitions for impel that I've seen before include "Egg" , "Make" , "Motivate" , "Goad" , "Force into motion" .)