Looking guilty about rat having not left a tip (8)

I believe the answer is:

gratuity

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'tip' is the definition.
(I know that tip can be written as gratuity)

I cannot understand how the remainder of the clue works.

(Other definitions for gratuity that I've seen before include "present" , "Tip (to waiter)" .)

I've seen this clue in the Sydney Morning Herald.

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