A French family eating some finally -- but it's dirty (7)
I believe the answer is:
unclean
'it's dirty' is the definition.
('unclean' can be a synonym of 'dirty')
'a french family eating some finally' is the wordplay.
'a french' becomes 'un' ('a' in French).
'family' becomes 'clan' (I've seen this before**).
'eating' indicates putting letters inside.
'finally' suggests the final letters.
The final letter of 'some' is 'e'.
'clan' enclosing 'e' is 'clean'.
'un'+'clean'='UNCLEAN'
'but' acts as a link.
(Other definitions for unclean that I've seen before include "Dirty or unwashed" , "taboo" , "Defiled" , "Ceremonially impure" , "Foul" .)