Descending bottom isn't keeping up (8,6)

I believe the answer is:

dropping behind

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'isn't keeping up' is the definition.
I can't judge whether this definition defines the answer.

'descending bottom' is the wordplay.
'descending' becomes 'dropping' (dropping is a kind of descending).
'bottom' becomes 'behind' (synonyms).
'dropping'+'behind'='DROPPING BEHIND'

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I've seen this clue in the Sydney Morning Herald.

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