Shy girl dressed in silk overnight (9,6)
I believe the answer is:
shrinking violet
'shy girl' is the definition.
'dressed in silk overnight' is the wordplay.
'dressed' indicates an anagram ('dress' can mean 'prepare').
'in'+'silk'+'overnight'='insilkovernight'
'insilkovernight' anagrammed gives 'SHRINKING VIOLET'.
(Other definitions for shrinking violet that I've seen before include "shy type" , "Exaggeratedly shy person" , "Plant not growing" , "wallflower" , "Person not likely to stand out" .)