Even out again when up to superficial coating (6)
I believe the answer is:
veneer
'superficial coating' is the definition.
(I've seen this before)
'even out again when up' is the wordplay.
'even out' becomes 'ven' (I can't explain this - if you can you should give a lot more credence to this answer).
'again' becomes 're' (prefix meaning 'again').
'when' becomes 'e' (I've seen this in other clues).
'up' says the letters should be written backwards (in down clue: letters go upwards).
're'+'e'='ree'
'ree' reversed gives 'eer'.
'ven'+'eer'='VENEER'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for veneer that I've seen before include "Thin layer glued to a surface" , "Thin wooden facing" , "Thin layer of a fine material on the surface" , "Thin covering on top, as of wood" , "'e never (anag.)" .)