Is the right size for almost stiff back (4)
I believe the answer is:
fits
'is the right size' is the definition.
('fit' can be a synonym of 'be the right size')
'almost stiff back' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'back' is a reversal indicator.
'stiff' with its last letter taken away is 'stif'.
'stif' back-to-front is 'FITS'.
'for' is the link.
(Other definitions for fits that I've seen before include "crises once" , "Prepares" , "Paroxysms" , "Is of the right shape and size" , "Seizures" .)