Is decisive (4)
I believe the answer is:
acts
'is decisive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acts that I've seen before include "Fifth NT book" , "Performances" , "New Testament book immediately following the Gospels" , "Behaves like thespian" , "Cast in major divisions of plays, like Shakespeare's" .)