PM, happy, put on weight (9)
I believe the answer is:
gladstone
'pm' is the definition.
(William Gladstone)
'happy put on weight' is the wordplay.
'happy' becomes 'glad' (similar in meaning).
'put on' says to put letters next to each other.
'weight' becomes 'stone' (stone is a kind of weight).
'glad'+'stone'='GLADSTONE'
(Other definitions for gladstone that I've seen before include "Former English PM or kind of bag" , "Old English P.M. or kind of bag" , "So tangled in a small bag like a doctor's" , "5,27 [PRIME MINISTER]" , "Victorian PM" .)