Declining and becoming sad (5,4)

I believe the answer is:

going down

'declining' is the definition.
('go down' can be a synonym of 'decline')

'becoming sad' is the wordplay.
'becoming' becomes 'going' ('go' can be a synonym of 'become').
'sad' becomes 'down' (down can mean sad or unhappy).
'going'+'down'='GOING DOWN'

'and' is the link.

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