Lazy person riled when exercising (5)
I believe the answer is:
idler
'lazy person' is the definition.
(I've seen this in another clue)
'riled when exercising' is the wordplay.
'when exercising' is an anagram indicator.
'riled' anagrammed gives 'IDLER'.
(Other definitions for idler that I've seen before include "One who doesn't want to work" , "papers" , "Person who does no work" , "One working to control tension" , "Lounger" .)