Do a gentle twist when stretched (9)
I believe the answer is:
elongated
'stretched' is the definition.
('elongate' can be a synonym of 'stretch')
'do a gentle twist' is the wordplay.
'twist' indicates anagramming the letters (a twisted or distorted version of the letters).
'do'+'a'+'gentle'='doagentle'
'doagentle' is an anagram of 'ELONGATED'.
'when' is the link.
(Other definitions for elongated that I've seen before include "Put on stretcher" , "Dangle toe (anag)" , "Stretched" , "Go al dente to be lengthened or prolonged" , "going further" .)