Master swimming in a small river (6)
I believe the answer is:
stream
'a small river' is the definition.
(I've seen this in another clue)
'master swimming' is the wordplay.
'swimming' indicates an anagram (letters swim around into a new order).
'master' anagrammed gives 'STREAM'.
'in' is the link.
(Other definitions for stream that I've seen before include "Natural body of running water" , "Small watercourse" , "Flock" , "group of pupils split by ability" , "Rill, brook" .)