Two members of family, one failing to finish crossword? Nonsense (5,5)
I believe the answer is:
mumbo jumbo
'nonsense' is the definition.
(I know that mumbo jumbo is a type of gibberish)
'two members of family one failing to finish crossword?' is the wordplay.
'two members of' becomes 'mum' (I can't justify this - if you can you should believe this answer much more).
'family' becomes 'box family' (box family is a kind of family).
'one failing to finish' means to remove the last letter.
'crossword?' becomes 'jumbo' (jumbo-sized crossword).
'box' with its last letter removed is 'bo'.
'mum'+'bo'+'jumbo'='MUMBO-JUMBO'
(Other definitions for mumbo jumbo that I've seen before include "Gobbledegook" , "Baffling jargon" , "obscure language" , "Mysterious speech" , "gibberish" .)