'K' is wrestling with Pat (sumo) (9)
I believe the answer is:
potassium
'k' is the definition.
('potassium' can be a synonym of 'k')
'is wrestling with pat sumo' is the wordplay.
'wrestling with' indicates anagramming the letters.
'pat'+'sumo'='patsumo'
'is' mixed up with 'patsumo' can make 'POTASSIUM'.
(Other definitions for potassium that I've seen before include "Reactive metal element" , "Element K" , "... is its atomic number: symbol K" , "Silvery metallic element" , "Soft metal, symbol K" .)