Girl wants a knight dressed in black (5)
I believe the answer is:
janet
'girl' is the definition.
(girl's name)
'a knight dressed in black' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'dressed in' indicates putting letters inside (inserted letters are dressed in others).
'black' becomes 'jet' (similar in meaning).
'a'+'n'='an'
'an' inserted into 'jet' is 'JANET'.
'wants' is the link.
(Other definitions for janet that I've seen before include "Female name" , "academic alliance?" , "Woman" .)