Agree never to bring up somewhat superficial appearance (6)
I believe the answer is:
veneer
'superficial appearance' is the definition.
(I've seen this before)
'agree never to bring up somewhat' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'to bring up' shows that the letters should be reversed in order (I've seen 'brought up' mean this (down clues only)).
'reenev' can be found hidden inside 'agree never'.
'reenev' in reverse letter order is 'VENEER'.
(Other definitions for veneer that I've seen before include "Fine overlay or superficial refinement" , "Thin decorative covering of fine wood" , "Thin coating on the outside" , "A thin outer layer" , "Thin layer of fine wood covering surface of cheaper wood" .)