Swimming riled lazybones (5)
I believe the answer is:
idler
'lazybones' is the definition.
(I've seen this in another clue)
'swimming riled' is the wordplay.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'riled' with letters rearranged gives 'IDLER'.
(Other definitions for idler that I've seen before include "Johnson's work" , "who makes no effort" , "Time waster" , "comparatively slack" , "One reluctant to work" .)