Lacking stiffness (4)
I believe the answer is:
limp
'lacking stiffness' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for limp that I've seen before include "Saggy and drooping" , "Lacking stiffness" , "Feeble" , "Flabby - halt" , "Walk with an injury" .)