A mate screening film occasionally shows resentment (8)
I believe the answer is:
acrimony
'resentment' is the definition.
(thesaurus)
'a mate screening film occasionally' is the wordplay.
'mate' becomes 'crony' (I've seen this before).
'screening' is an insertion indicator (I've seen this in other clues).
'occasionally' means one should take alternating letters.
The alternate letters of 'film' are 'im'.
'crony' enclosing 'im' is 'crimony'.
'a'+'crimony'='ACRIMONY'
'shows' is the link.
(Other definitions for acrimony that I've seen before include "Angry and unpleasant feelings or words" , "I cry, moan (anag)" , "Acronym I made out of bitterness" , "Bitterness, ill feeling" , "Bitterness or ill-feeling" .)