When skin turned numb (6)
I believe the answer is:
asleep
'numb' is the definition.
(I've seen this before)
'when skin turned' is the wordplay.
'when' becomes 'as'.
'skin' becomes 'peel' (synonyms).
'turned' shows that the letters should be reversed in order.
'peel' backwards is 'leep'.
'as'+'leep'='ASLEEP'
(Other definitions for asleep that I've seen before include "coiled up?" , "Having forty winks" , "Knitting up the ravelled sleeve of care" , "Like kippers" , "Inattentive; numbed" .)