Bill confused with A-minus being hidden (10)

I believe the answer is:

subliminal

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'hidden' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I don't see?

'bill confused with a-minus' is the wordplay.
'confused with' indicates an anagram.
'a'+'minus'='aminus'
'bill' with its letters mixed up with 'aminus' gives 'SUBLIMINAL'.

'being' is the link.

Can you help me to learn more?

(Other definitions for subliminal that I've seen before include "Below the conscious level" , "sort of advertisement" .)

I've seen this clue in The Guardian.

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