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