Girl one's crazy about (4)
I believe the answer is:
maid
'girl' is the definition.
(I've seen this in other clues)
'one's crazy about' is the wordplay.
'one' becomes 'i' (Roman numeral).
'crazy' becomes 'mad' (synonyms).
'about' means one lot of letters goes inside another.
'i' put inside 'mad' is 'MAID'.
(Other definitions for maid that I've seen before include "Damsel" , "Female domestic servant" , "Serving girl" , "daughter?" , "Young woman - servant" .)