Presume swimming is best (7)
I believe the answer is:
supreme
'is best' is the definition.
(I've seen this before)
'presume swimming' is the wordplay.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'presume' anagrammed gives 'SUPREME'.
(Other definitions for supreme that I've seen before include "Best of all" , "Chief" , "Highest in rank - incomparable" , "Creamy sauce" , "topmost level" .)