What could be laciest piece of stretchy material (7)
I believe the answer is:
elastic
'piece of stretchy material' is the definition.
(elastic is stretchy material)
'what could be laciest' is the wordplay.
'what could be' indicates an anagram.
'laciest' with letters rearranged gives 'ELASTIC'.
(Other definitions for elastic that I've seen before include "Able to stretch" , "Flexible, adaptable" , "Cats lie (anag.)" , "Ductile" , "Giving" .)