About to make a mistake, wearing sensible white (8)
I believe the answer is:
sancerre
'white' is the definition.
(I've seen this before)
'about to make a mistake wearing sensible' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to make a mistake' becomes 'err' (to err is to make an error).
'wearing' means one lot of letters goes inside another.
'sensible' becomes 'sane' (sane can mean sensible or reasonable).
'c'+'err'='cerr'
'cerr' put inside 'sane' is 'SANCERRE'.
(Other definitions for sancerre that I've seen before include "Type of French white wine" , "Light French white wine" .)