Accessory in shock admission, initially: Queen, possibly, has stolen large diamond (5,4)
I believe the answer is:
alice band
'accessory in shock' is the definition.
(type of hair band - shock can mean a mass of hair)
'admission initially queen possibly has stolen large diamond' is the wordplay.
'initially' suggests taking the first letters.
'queen possibly' becomes 'band' (Queen are a band).
'has stolen' means one lot of letters goes inside another (inserted letters are stolen).
'large' becomes 'L' (eg in clothes sizes).
'diamond' becomes 'ice' (ice can informally mean diamonds).
The first letter of 'admission' is 'a'.
'a'+'band'='aband'
'l'+'ice'='lice'
'aband' enclosing 'lice' is 'ALICE BAND'.
(Other definitions for alice band that I've seen before include "It's worn to hold back the hair" , "that contains shock?" , "Lock keeper" , "headwear" , "hair ribbon" .)