Good chaps with sound Prime Minister (9)
I believe the answer is:
gladstone
'prime minister' is the definition.
(William Ewart Gladstone Prime Minister 1868-1894)
'good chaps with sound' is the wordplay.
'good' becomes 'g' (abbreviation).
'chaps' becomes 'lads' ('lad' can be a synonym of 'chap').
'with' is a charade indicator (letters next to each other).
'sound' becomes 'tone' (tone is a kind of sound**).
'g'+'lads'+'tone'='GLADSTONE'
(Other definitions for gladstone that I've seen before include "So tangled up with old British Prime Minister" , "5,27 [PRIME MINISTER]" , "British statesman, four times PM, d. 1898" , "Bag for a former British statesman" , "Old English P.M. or kind of bag" .)