Stretches, so elegant, when exercising (9)
I believe the answer is:
elongates
'stretches' is the definition.
('elongate' can be a synonym of 'stretch')
'so elegant when exercising' is the wordplay.
'when exercising' indicates an anagram.
'so'+'elegant'='soelegant'
'soelegant' anagrammed gives 'ELONGATES'.
(Other definitions for elongates that I've seen before include "Extends" , "pulls out" , "tortures on the rack!" , "lengthens the process" , "stretches" .)