PM's meagre salary halved (4)
I believe the answer is:
pitt
'pm's' is the definition.
(prime minister William Pitt)
'meagre salary halved' is the wordplay.
'meagre salary' becomes 'pittance' (as in being paid a pittance).
'halved' suggests halving the letters.
'pittance' with half the letters taken is 'PITT'.
(Other definitions for pitt that I've seen before include "Brad --, US actor" , "William --, early PM" , "One of two Tory prime ministers" , "Former Prime Minister" , "William - the Younger, twice prime minister" .)