Cushion sags at the back again and again (6)
I believe the answer is:
soften
'cushion' is the definition.
(synonyms)
'sags at the back again and again' is the wordplay.
'at the back' suggests the final letters (the letter at the back).
'again and again' becomes 'often' (I've seen this before).
The last letter of 'sags' is 's'.
's'+'often'='SOFTEN'
(Other definitions for soften that I've seen before include "To moderate" , "Make more plastic" , "Make less firm" , "Yield" , "become less rough" .)