Pitiless truth represented in film (3-6)
I believe the answer is:
cut-throat
'pitiless' is the definition.
'truth represented in film' is the wordplay.
'represented' indicates anagramming the letters.
'in' is an insertion indicator.
'film' becomes 'coat' (I've seen this in other clues**).
'truth' anagrammed gives 'utthr'.
'utthr' going inside 'coat' is 'CUT-THROAT'.
(Other definitions for cut-throat that I've seen before include "Fiercely competitive - type of razor" , "Murderer - fiercely competitive" , "Violent offender" , "Ruthless in competition" .)