Clue: Former prime minister's upset, ultimately very tired (6)
Answer: SLEEPY
Explanation: Definition: tired
A charade of SLEEP, a reversal ('upset' – more common in a down light, but not here) of PEEL'S (Sir Robert, 'former Prime Minister's' – quite a bit former, 1834-35 and 1841-46); plus Y ('ultimately ver**Y**').