I'm impressed the French almost lied about checks (2,4)
I believe the answer is:
no less
'i'm impressed' is the definition.
I can't tell whether this definition defines the answer.
'the french almost lied about checks' is the wordplay.
'the french' becomes 'les' (plural form of 'the' in French).
'almost' means to remove the last letter (most of the word but not all of it).
'lied' becomes 'song' ('Lied' is German for song).
'about' shows that the letters should be reversed in order.
'checks' means one lot of letters goes inside another (inserted letters are checked or controlled).
'song' with its last letter taken away is 'son'.
'son' back-to-front is 'nos'.
'les' placed within 'nos' is 'NO LESS'.
Can you help me to learn more?
(Other definitions for no less that I've seen before include "Bare minimum" , "Phrase used to express admiration - lesson (anag)" , "as important" , "would you believe!" , "Phrase used to emphasise admiration or surprise" .)