Good chap's pitch for Prime Minister (9)
I believe the answer is:
gladstone
'for prime minister' is the definition.
(William Ewart Gladstone Prime Minister 1868-1894)
'good chap's pitch' is the wordplay.
'good' becomes 'g' (abbreviation).
'chap's' becomes 'lad's'.
'pitch' becomes 'tone' (tone is a kind of pitch).
'g'+'lads'+'tone'='GLADSTONE'
(Other definitions for gladstone that I've seen before include "Old English P.M. or a doctor's bag" , "Tangled so like hinged case" , "Victorian GOM" , "Victorian PM" , "C. nineteenth Liberal prime minister" .)