Suitable for a bit of a stiff back (3)
I believe the answer is:
fit
'suitable' is the definition.
(similar in meaning)
'a bit of a stiff back' is the wordplay.
'a bit of' indicates a hidden word.
'back' says the letters should be written backwards.
'tif' is hidden within 'stiff'.
'tif' in reverse letter order is 'FIT'.
'for' acts as a link.
(Other definitions for fit that I've seen before include "Certified OK by the doctor" , "Adapt; well" , "Healthy; conform to" , "Suitable; healthy" , "Strong" .)