When skin turns numb (6)
I believe the answer is:
asleep
'numb' is the definition.
(I've seen this before)
'when skin turns' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'skin' becomes 'peel' (synonyms).
'turns' says the letters should be written in reverse.
'peel' reversed gives 'leep'.
'as'+'leep'='ASLEEP'
(Other definitions for asleep that I've seen before include "Slumbering" , "Please be at rest, oddly" , "Dead to the world" , "not paying attention" , "like a kipper" .)