Cushion beginning to sag regularly (6)
I believe the answer is:
soften
'cushion' is the definition.
(synonyms)
'beginning to sag regularly' is the wordplay.
'beginning to' says to take the initial letters.
'regularly' becomes 'often' (I've seen this before).
The initial letter of 'sag' is 's'.
's'+'often'='SOFTEN'
(Other definitions for soften that I've seen before include "become less rough" , "Tone down" , "Become gentler" , "Mitigate" , "Make less firm" .)