A girl's crazy about one (4)

Here is my best explanation:

'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'.