A post on the motorway discovered in error (7)
I believe the answer is:
mistake
'error' is the definition.
(I know that error can be written as mistake)
'a post on the motorway discovered in' is the wordplay.
'a post' becomes 'stake' (synonyms).
'on' is a charade indicator (letters next to each other).
'the motorway' becomes 'M' (as in motorway names - Ms, Mk etc).
'discovered' means to look at the middle letters (dis-covered - outer letters removed).
'in' becomes 'hip' (both can mean fashionable).
The middle letter of 'hip' is 'i'.
'm'+'i'='mi'
'stake' put after 'mi' is 'MISTAKE'.
(Other definitions for mistake that I've seen before include "Slip" , "16 down" , "Oversight" , "Fault" , "Wrong action or judgment" .)