Reinforce a cushion (7)
I believe the answer is:
bolster
I believe this clue is a double definition.
'reinforce' is the first definition.
(bolstering is a kind of reinforcing)
'a cushion' is the second definition.
(bolster is a kind of cushion)
(Other definitions for bolster that I've seen before include "Provide reinforcement" , "Lobster (anag.)" , "Hold up" , "Long thick pillow" , "Strengthen - long pillow" .)