A function that begins explanation informally? (3)

I believe the answer is:

cos

I believe this clue is a double definition.

'a function' is the first definition.
(mathematical function)

'that begins explanation informally?' is the second definition.
(shortened pronunciation of 'because')

(Other definitions for cos that I've seen before include "Senior officers" , "leaves > function" , "Kind of lettuce in microcosm" , "Variety of lettuce" , "Salad ingredient" .)

I've seen this clue in The Times.

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