Restrain pet (6)
I believe the answer is:
temper
This clue is a double definition.
'restrain' is the first definition.
(thesaurus)
'pet' is the second definition.
('pet' can mean a tantrum)
(Other definitions for temper that I've seen before include "Rage" , "Tendency to anger" , "Moderate; ill humour" , "State of mind as regards anger (6)" , "Moderate - mood" .)