Add screening about to show nasty film (4)
I believe the answer is:
scum
'nasty film' is the definition.
(I've seen this in another clue)
'add screening about' is the wordplay.
'add' becomes 'sum' (synonyms).
'screening' is an insertion indicator (inserted letters are covered or screened by others).
'about' becomes 'c' (circa).
'sum' enclosing 'c' is 'SCUM'.
'to show' acts as a link.
(Other definitions for scum that I've seen before include "Impurities layer" , "Floating film of filth" , "Floating impurity layer" , "Matter floating on 9 ac" , "Waste matter floating on the surface" .)