Blatant imposture girl and boy must defend (9)
I believe the answer is:
unashamed
'blatant' is the definition.
(I've seen this before)
'imposture girl and boy must defend' is the wordplay.
I cannot quite understand how this works, but
'imposture' could be 'sham' (I've seen this in another clue) and 'sham' is found in the answer.
'girl' could be 'una' (girl's name) and 'una' is found within the answer.
'boy' could be 'ed' (short for Edward) and 'ed' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for unashamed that I've seen before include "Unabashed" , "Blatant" , "Not at all abashed or penitent" , "Without guilt or embarrassment" , "Brazen" .)