Unbending line bending when given little jerk (9)
I believe the answer is:
inelastic
'unbending' is the definition.
(thesaurus)
'line bending when given little jerk' is the wordplay.
'bending' indicates an anagram.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'given' is a charade indicator (letters next to each other).
'little jerk' becomes 'tic' (jerking movement by the body).
'line' is an anagram of 'inel'.
'inel'+'as'+'tic'='INELASTIC'
(Other definitions for inelastic that I've seen before include "Unyielding" , "Non-stretch" , "Broken" , "stiff" , "Having no give" .)