Scorch or burn the surface of (4)
I believe the answer is:
sear
I believe this is a double definition.
'scorch' is the first definition.
(I know that scorch can be written as sear)
'burn the surface of' is the second definition.
(I've seen this before)
(Other definitions for sear that I've seen before include "Burn, scorch intensely" , "Scorch or burn the surface of" , "Burn briefly" , "Cauterize" , "Scorch, brand" .)