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