Wheat's second skin? (5)
I believe the answer is:
spelt
'wheat's' is the definition.
(spelt is a kind of wheat)
'second skin?' is the wordplay.
'second' becomes 's' (common abbreviation).
'skin?' becomes 'pelt' (pelt is a kind of skin).
's'+'pelt'='SPELT'
(Other definitions for spelt that I've seen before include "cereal" , "An old type of wheat" , "Given letter by letter; wheat" , "Recited the letters of a word" , "Made up a word" .)