Flexible seal shattered with little jerk (7)
I believe the answer is:
elastic
'flexible' is the definition.
(I know that flexible can be written as elastic)
'seal shattered with little jerk' is the wordplay.
'shattered' indicates an anagram.
'with' means one lot of letters go next to another.
'little jerk' becomes 'tic' (jerking movement by the body).
'seal' is an anagram of 'elas'.
'elas'+'tic'='ELASTIC'
(Other definitions for elastic that I've seen before include "Castile (anag)" , "Scale it (anag.)" , "Resilient" , "Material for bands" , "Stretching tape" .)