A former PM's climbing out (6)
I believe the answer is:
asleep
'out' is the definition.
(out can mean unconscious or asleep)
'a former pm's climbing' is the wordplay.
'a former' becomes 'as' (I can't explain this - if you can you should give a lot more credence to this answer).
'pm' becomes 'Peel' (a9th-century prime minister Robert Peel).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'peel' reversed gives 'leep'.
'as'+'leep'='ASLEEP'
Can you help me to learn more?
(Other definitions for asleep that I've seen before include "Having nodded off" , "Inattentive; numbed" , "Resting unconscious" , "Please be at rest (6)." , "out like a light?" .)