Crazy about one young girl (4)
I believe the answer is:
maid
'young girl' is the definition.
(I've seen this in another clue)
'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 "Young woman - servant" , "Female servant - young girl" , "One in service" , "Female domestic servant" , "Serving girl" .)