Not stiff or firm (4)
I believe the answer is:
limp
'not stiff or firm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for limp that I've seen before include "obstacle to smooth progress" , "Walk badly; not stiff" , "Walk with an uneven step" , "Weak" , "Flabby - halt" .)