Feel a stickler will keep flexible (7)
I believe the answer is:
elastic
'flexible' is the definition.
('elastic' can be similar in meaning to 'flexible')
'feel a stickler will keep' is the wordplay.
'keep' indicates the answer is hidden within the clue.
'ELASTIC' can be found hidden inside 'feel a stickler will'.
(Other definitions for elastic that I've seen before include "Stretching tape" , "Cats lie (anag.)" , "Fluid" , "Stretchable material" , "Tape containing flexible rubber" .)