Not keeping up with sinking bottom (8,6)

I believe the answer is:

dropping behind

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

'with sinking bottom' is the wordplay.
'with' means one lot of letters go next to another.
'sinking' becomes 'dropping' ('drop' can be a synonym of 'sink').
'bottom' becomes 'behind' (synonyms).
'dropping' put next to 'behind' is 'DROPPING BEHIND'.

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

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