Actress upset about start of play with constant sources of alarm (5,7)
I believe the answer is:
scare tactics
'sources of alarm' is the definition.
'actress upset about start of play with constant' is the subsidiary indication.
'upset' indicates anagramming the letters.
'about' is an insertion indicator.
'start of play' becomes 'acti' (Act I).
'with' says to put letters next to each other.
'constant' becomes 'c' (mathematical abbreviation).
'actress' anagrammed gives 'scarets'.
'acti'+'c'='actic'
'scarets' enclosing 'actic' is 'scaretactics'.
(Other definitions for scare tactics that I've seen before include "Action plan intended to intimidate" , "Actions solely intended to intimidate" .)