Former prime minister's upset, ultimately very tired (6)

I believe the answer is:

sleepy

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'tired' is the definition.
(falling asleep)

'former prime minister's upset ultimately very' is the wordplay.
'former' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'prime minister' becomes 'peel' (Robert Peel).
'upset' is a reversal indicator.
'ultimately' says to take the final letters.
The last letter of 'very' is 'y'.
'peel' back-to-front is 'leep'.
's'+'leep'+'y'='SLEEPY'

Can you help me to learn more?

(Other definitions for sleepy that I've seen before include "Doc's nodding pal" , "(Of place) without much activity" , "Soporific" , "Is inactive" , "Ready to turn in" .)

I've seen this clue in The Guardian.

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