Exceptionally vain girl's instrument (8)
I believe the answer is:
virginal
'instrument' is the definition.
(virginal is a kind of instrument)
'exceptionally vain girl's' is the wordplay.
'exceptionally' indicates anagramming the letters.
'vain'+'girl'='vaingirl'
'vaingirl' is an anagram of 'VIRGINAL'.
(Other definitions for virginal that I've seen before include "Pure - very early keyboard instrument" , "Antique keyboard" , "spotless" , "Untouched - old keyboard instrument" , "Unblemished" .)