A large diamond for adventurous girl (5)
I believe the answer is:
alice
'adventurous girl' is the definition.
(main character in Alice's Adventures in Wonderland)
'a large diamond' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'diamond' becomes 'ice' (ice can informally mean diamonds).
'a'+'l'+'ice'='ALICE'
'for' is the link.
(Other definitions for alice that I've seen before include "Wondergirl" , "She fell down a rabbit hole into a fantasy world" , "Celia changes into a different girl" , "woman" , "Carroll heroine" .)