Crossword Genius

Henry not attending club function (3)

Ross

I believe the answer is:

cos

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'function' is the definition.
(mathematical function)

I don't understand how the rest of the clue works.

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(Other definitions for cos that I've seen before include "Greek island and type of lettuce" , "providing unsatisfactory explanation?" , "Salad plant" , "Kind of lettuce in microcosm" , "foreign island" .)

I've seen this clue in The Independent.
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