Girl one's crazy about (4)
I believe the answer is:
maid
'girl' is the definition.
(I know that maid is a type of girl)
'one's crazy about' is the wordplay.
'one' becomes 'i' (Roman numeral).
'crazy' becomes 'mad' (synonyms).
'about' indicates putting letters inside.
'i' going into 'mad' is 'MAID'.
(Other definitions for maid that I've seen before include "Damsel" , "Female domestic servant" , "Serving girl" , "daughter?" , "Young woman - servant" .)