Annoyance over horrible mess, botch, ultimately result of PM’s decision? (9)
I believe the answer is:
reshuffle
'result of pm's decision?' is the definition.
(as in a prime minister reshuffling the cabinet)
'annoyance over horrible mess botch ultimately' is the wordplay.
'annoyance' becomes 'ruffle' (ruffle can mean a disturbance or annoyance).
'over' indicates putting letters inside.
'ultimately' says to take the final letters.
The last letters of 'horrible mess botch' are 'esh'.
'ruffle' placed around 'esh' is 'RESHUFFLE'.
(Other definitions for reshuffle that I've seen before include "Change members of Government" , "Change of government jobs" , "Recycle old cabinet" , "Cabinet musical chairs" , "Cabinet rearrangement" .)