Prime minister pleased to put on some weight (9)
I believe the answer is:
gladstone
'prime minister' is the definition.
(William Ewart Gladstone Prime Minister 1868-1894)
'pleased to put on some weight' is the wordplay.
'pleased' becomes 'glad' (similar in meaning).
'to put on' says to put letters next to each other.
'some weight' becomes 'stone' (stone is a unit of weight).
'glad'+'stone'='GLADSTONE'
(Other definitions for gladstone that I've seen before include "C. nineteenth Liberal prime minister" , "So tangled in a small bag like a doctor's" , "PM no longer" , "Former English PM or kind of bag" , "Tangled so like hinged case" .)