Boss opening theatre in south of France (4)
I believe the answer is:
stud
'boss' is the definition.
(both can mean a metal decoration)
'opening theatre in south of france' is the wordplay.
'opening' indicates taking the first letters.
'in' indicates putting letters inside.
'south of france' becomes 'sud' (French for south).
The first letter of 'theatre' is 't'.
't' put into 'sud' is 'STUD'.
(Other definitions for stud that I've seen before include "Ladies' man?" , "Large-headed nail" , "A lot of horses" , "Horse-breeding establishment; jewellery item" , "Horse-breeding farm" .)