Vacant secretariat, a politician’s hallmark! (5)
I believe the answer is:
stamp
'hallmark' is the definition.
(I've seen this before)
'vacant secretariat a politician's' is the wordplay.
'vacant' suggests removing the centre.
'politician' becomes 'mp' (Member of Parliament).
'secretariat' with its centre removed is 'st'.
'st'+'a'+'mp'='STAMP'
(Other definitions for stamp that I've seen before include "cast?" , "Official mark; bang (foot)" , "Bring one's foot down" , "Bring foot down hard" , "contributions?" .)