Dance king has put on weight! (10)
I believe the answer is:
charleston
'dance' is the definition.
(charleston is a kind of dance)
'king has put on weight' is the wordplay.
'king has' becomes 'charles' (I can't explain this - if you can you should believe this answer much more).
'put on' means one lot of letters go next to another.
'weight' becomes 'ton' (unit of weight).
'charles'+'ton'='CHARLESTON'
Can you help me to learn more?
(Other definitions for charleston that I've seen before include "Town of South Carolina which gave its name to a 1920s dance craze" , "US city - 1920s' dance" , "American port" , "Nineteen twenties dance" , "Dance making" .)