Familiar with a universal force on small island (2,4)
I believe the answer is:
au fait
'familiar' is the definition.
(I've seen this before)
'a universal force on small island' is the wordplay.
'universal' becomes 'U' (film certificate).
'force' becomes 'f' (scientific abbreviation).
'on' means one lot of letters go next to another.
'small island' becomes 'ait' (an ait is a small island in a river).
'a'+'u'+'f'+'ait'='AU FAIT'
'with' acts as a link.
(Other definitions for au fait that I've seen before include "Well acquainted with a subject - in France" , "Familiar with - in France" , "Well-informed" , "Conversant or well acquainted with - in France" , "Up to speed (from French)" .)