A small layer of a certain type of wood (5)
I believe the answer is:
ashen
'a certain type of wood' is the definition.
I can't tell whether this definition defines the answer.
'a small layer' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'layer' becomes 'hen' (something which lays - eggs in this case).
'a'+'s'+'hen'='ASHEN'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for ashen that I've seen before include "Pale (with shock/illness)" , "So vampire might appear" , "'Very pale, from shock say (5)'" , "Drained of colour" , "Deathly pale" .)