Able to help in stiffening up (3)
I believe the answer is:
fit
'able' is the definition.
(similar in meaning)
'in stiffening up' is the wordplay.
'in' indicates a hidden word.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'tif' is hidden in the letters of 'stiffening'.
'tif' written backwards gives 'FIT'.
'to help' acts as a link.
(Other definitions for fit that I've seen before include "Strong" , "Be the right size or shape" , "In good physical shape 93)" , "Suitable; healthy" , "Sound in body or suitable" .)