Expose man's error in Britain (6)
I believe the answer is:
unmask
'expose' is the definition.
(I know that unmask is a more specific form of expose)
'man's error in britain' is the wordplay.
'error' is an anagram indicator.
'in' means one lot of letters goes inside another.
'britain' becomes 'uk' (synonyms).
'mans' is an anagram of 'nmas'.
'nmas' going inside 'uk' is 'UNMASK'.
(Other definitions for unmask that I've seen before include "Expose, remove face covering" , "Discover the identity of" , "Reveal, expose" , "Expose, strip away concealment" , "Expose and denounce" .)