Film showing key in calculation (4)
I believe the answer is:
scum
'film' is the definition.
(I know that scum is a type of film)
'key in calculation' is the wordplay.
'key' becomes 'c' (musical key).
'in' indicates putting letters inside.
'calculation' becomes 'sum' (I have seen 'Arithmetic calculation ' mean 'sum' so perhaps 'calculation' could also mean 'sum').
'c' inserted inside 'sum' is 'SCUM'.
'showing' acts as a link.
(Other definitions for scum that I've seen before include "Floating film of filth" , "Impurities that rise to surface of liquid" , "Worthless man" , "Dross" , "Impurities layer" .)