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' is an insertion indicator.
'i' inserted within 'mad' is 'MAID'.
(Other definitions for maid that I've seen before include "Damsel" , "Female domestic servant" , "Serving girl" , "daughter?" , "Young woman - servant" .)