Policeman in Boris's first campaign sacking 50 (5)
I believe the answer is:
bobby
'policeman' is the definition.
(bobby is a kind of policeman)
'boris's first campaign sacking 50' is the wordplay.
'boris's first' becomes 'b' (1st letter of 'boris').
'campaign' becomes 'lobby' (as in lobbying for a political goal).
'sacking' suggests deleting specific letters.
'50' becomes 'l' (Roman numeral).
'lobby' with 'l' taken out is 'obby'.
'b'+'obby'='BOBBY'
'in' acts as a link.
(Other definitions for bobby that I've seen before include "Rob" , "-- Moore; -- Charlton (footballers)" , "- Moore, World Cup winning captain" , "Policeman (sl.)" , "beat man" .)