Policeman to propose holding most of detectives up (7)
I believe the answer is:
officer
'policeman' is the definition.
('officer' can be a synonym of 'policeman')
'propose holding most of detectives up' is the wordplay.
'propose' becomes 'offer' (offering is a kind of proposing**).
'holding' indicates putting letters inside.
'most of' means to remove the last letter.
'detectives' becomes 'cia'.
'up' is a reversal indicator (in down clue: letters go upwards).
'cia' with its last letter taken off is 'ci'.
'ci' back-to-front is 'ic'.
'offer' going around 'ic' is 'OFFICER'.
'to' acts as a link.
(Other definitions for officer that I've seen before include "He holds a commission" , "9 across" , "Member of police force" , "Serviceman (or woman) of rank" , "Maybe society treasurer" .)