Not alert, a PM's making a U-turn (6)
I believe the answer is:
asleep
'not alert' is the definition.
(I've seen this in another clue)
'a pm's making a u-turn' is the wordplay.
'a pm's' becomes 'Peels' (a9th-century prime minister Robert Peel).
'making' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'a u-turn' shows that the letters should be reversed in order.
'peels'+'a'='peelsa'
'peelsa' back-to-front is 'ASLEEP'.
Can you help me to learn more?
(Other definitions for asleep that I've seen before include "Having forty winks" , "Out for the count" , "Having nodded off" , "in bed?" , "Dead to the world" .)