Crossword Genius

With a flap, it's cooler (4)

Ross

I believe the answer is:

stir

I'm a little stuck... Click here to teach me more about this clue!

'cooler' is the definition.
(both are slang terms for prison)

'with a flap it's' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'sir' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for stir that I've seen before include "Budge" , "Thrill" , "Disturb" , "Get moving" , "Whisk or whip" .)

I've seen this clue in the Evening Standard.
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