A girl's crazy about one (4)
I believe the answer is:
maid
'a girl's' is the definition.
(maid is a kind of girl)
'crazy about one' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'mad' going around 'i' is 'MAID'.
(Other definitions for maid that I've seen before include "Female servant - young girl" , "Young woman - servant" , "Female domestic servant" , "One in service" , "- - -of-all-work" .)