Volunteer to take in unprotected sick policeman (7)
I believe the answer is:
officer
'policeman' is the definition.
('officer' can be a synonym of 'policeman')
'volunteer to take in unprotected sick' is the wordplay.
'volunteer' becomes 'offer' ('offer' can be a synonym of 'volunteer').
'to take in' indicates putting letters inside.
'unprotected' indicates the central letters.
The centre of 'sick' is 'ic'.
'offer' placed around 'ic' is 'OFFICER'.
(Other definitions for officer that I've seen before include "Public servant" , "eg Colonel" , "Leading soldier" , "Responsible type" , "Possibly general" .)