Mixing English in sign a film's unsuitable for kids? (12)
I believe the answer is:
adulterating
'mixing' is the definition.
(to adulterate is to mix with something else)
'english in sign a film's unsuitable for kids?' is the wordplay.
'english' becomes 'E' (abbreviation).
'in' is an insertion indicator.
'sign a film's unsuitable for kids?' becomes 'adultrating' (rating show a film is for adults only).
'e' going into 'adultrating' is 'ADULTERATING'.
(Other definitions for adulterating that I've seen before include "Making less pure" , "Debasing" .)