Lace -- it's unusually stretchy (7)
I believe the answer is:
elastic
'stretchy' is the definition.
('elastic' can be similar in meaning to 'stretchy')
'lace it's unusually' is the wordplay.
'unusually' indicates anagramming the letters.
'lace'+'its'='laceits'
'laceits' is an anagram of 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Material for bands" , "Flexible, adaptable" , "Like one across" , "Able to give a bit" , "Tape containing flexible rubber" .)