Policeman in command enters proposal (7)
I believe the answer is:
officer
'policeman' is the definition.
(synonyms)
'in command enters proposal' is the wordplay.
'in command' becomes 'ic' (military abbreviation).
'enters' is an insertion indicator.
'proposal' becomes 'offer' (I've seen this before).
'ic' placed into 'offer' is 'OFFICER'.
(Other definitions for officer that I've seen before include "Person holding authoritative position" , "Military person in authority" , "One with commission" , "Person holding a commission in the services" , "Holder of responsible position" .)