The first aria about some jewellery (5)
I believe the answer is:
tiara
'some jewellery' is the definition.
(tiara is a type of jewellery)
'the first aria about' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'about' indicates anagramming the letters.
'aria' with letters rearranged gives 'iara'.
't'+'iara'='TIARA'
(Other definitions for tiara that I've seen before include "High diadem" , "Small crown" , "Woman's headdress for very formal occasions" , "Pope's triple-tiered crown" , "Ornamental head gear" .)