Volunteers guarding international artist's jewellery (5)
I believe the answer is:
tiara
'jewellery' is the definition.
(tiara is a type of jewellery)
'volunteers guarding international artist's' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'guarding' indicates putting letters inside.
'international' becomes 'i' (abbreviation eg in IMF).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'ta' enclosing 'i' is 'tia'.
'tia'+'ra'='TIARA'
(Other definitions for tiara that I've seen before include "Women's head ornament" , "it goes to one's head!" , "Decorative headgear" , "Jewelled crescent-shaped headdress" , "Top decoration" .)