Knot in lace - it's stiff (9)
I believe the answer is:
inelastic
'stiff' is the definition.
(I've seen this before)
'knot in lace it's' is the wordplay.
'knot' indicates anagramming the letters (I've seen 'knotted' mean this).
'in'+'lace'+'its'='inlaceits'
'inlaceits' is an anagram of 'INELASTIC'.
(Other definitions for inelastic that I've seen before include "Without a spring" , "Rigid" , "stiff" , "Unyielding" , "Having no give" .)