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 the action expose)
'man's error in britain' is the wordplay.
'error' indicates anagramming the letters.
'in' is an insertion indicator.
'britain' becomes 'uk' (synonyms).
'mans' with letters rearranged gives 'nmas'.
'nmas' inserted within '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" .)