Board not coming up for food (8)

I believe the answer is:

plankton

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'food' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'board not coming up' is the wordplay.
'board' becomes 'plank' (synonyms).
'coming up' says the letters should be written backwards (in a down clue, letters go up).
'not' reversed gives 'ton'.
'plank'+'ton'='PLANKTON'

'for' acts as a link.

Can you help me to learn more?

(Other definitions for plankton that I've seen before include "Whale's staple diet" , "Aggregate of small marine organisms" , "Drifters in the sea" , "Drifting marine organisms" , "Water micro-organisms" .)

I've seen this clue in The Irish Times.

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