Show how un-English is honest criticism (4,7)
I believe the answer is:
fair comment
'honest criticism' is the definition.
(I've seen this before)
'show how un-english' is the wordplay.
'show' becomes 'fair' (fair is a kind of show).
'how un-english' becomes 'comment' ('how' in French).
'fair'+'comment'='FAIR COMMENT'
'is' acts as a link.
(Other definitions for fair comment that I've seen before include "Nothing libellous" , "Honest criticism" .)