Stretching green mantle is out of order (11)
I believe the answer is:
enlargement
'stretching' is the definition.
(thesaurus)
'green mantle is out of order' is the wordplay.
'is out of order' indicates anagramming the letters.
'green'+'mantle'='greenmantle'
'greenmantle' is an anagram of 'ENLARGEMENT'.
(Other definitions for enlargement that I've seen before include "Photographic blow-up" , "Bigger version of photographic print" , "increase" , "Blown-up picture" , "Blown-up photo" .)