Thinner veil revealing cheek (5)
I believe the answer is:
nerve
'cheek' is the definition.
(I've seen this before)
'thinner veil revealing' is the wordplay.
'revealing' indicates a hidden word.
'NERVE' can be found hidden inside 'thinner veil'.
(Other definitions for nerve that I've seen before include "presumption" , "Courage - or effrontery" , "Bundle of fibres in the body (5)" , "Fibre that carries instructions to the brain" , "Fibre carrying impulses from brain to body" .)