Crossword Genius

'Cause you can eat it! (3)

Ross

I believe the answer is:

cos

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

I cannot understand how the rest of the clue works.

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(Other definitions for cos that I've seen before include "Chief of Staff" , "Kind of lettuce in microcosm" , "often associated with sin" , "providing unsatisfactory explanation?" , "healthy food" .)

I've seen this clue in the Evening Standard.
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