Fortitude after local anaesthetic at the dentists? (5,5,3)

I believe the answer is:

stiff upper lip

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'fortitude' is the definition.
(I have seen 'Calm fortitude ' mean 'stiff upper lip' so perhaps 'fortitude' could also mean 'stiff upper lip')

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

Can you help me to learn more?

(Other definitions for stiff upper lip that I've seen before include "Very British stoicism" , "Symbol of resolution in bad times" , "Quality of stoicism" , "Calm courage" , "Fortitude in adversity" .)

I've seen this clue in The Irish Independent.

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