Policeman in charge introduced to volunteer (7)
I believe the answer is:
officer
'policeman' is the definition.
('officer' can be a synonym of 'policeman')
'in charge introduced to volunteer' is the wordplay.
'in charge' becomes 'ic'.
'introduced to' is an insertion indicator.
'volunteer' becomes 'offer' ('offer' can be a synonym of 'volunteer').
'ic' put within 'offer' is 'OFFICER'.
(Other definitions for officer that I've seen before include "Holder of a post" , "Maybe society treasurer" , "Policeman (or other authority)" , "Commissioned rank" , "Commissioned serviceman" .)