White layer beneath exposed mass (5)
I believe the answer is:
ashen
'white' is the definition.
(I've seen this before)
'layer beneath exposed mass' is the wordplay.
'layer' becomes 'hen' (hens lay eggs).
'beneath' means one lot of letters go next to another (in a down clue).
'exposed' says to take the centre (the inside of the word is exposed).
The middle of 'mass' is 'as'.
'hen' after 'as' is 'ASHEN'.
(Other definitions for ashen that I've seen before include "may be pale with shock" , "Grey, pallid" , "Pale (with shock/illness)" , "So vampire might appear" , "Anaemic-looking" .)