Servant is crazy about one (4)
I believe the answer is:
maid
'servant' is the definition.
(I know that maid is a type of domestic)
'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'.
'is' acts as a link.
(Other definitions for maid that I've seen before include "One in service" , "Serving girl" , "- - -of-all-work" , "Damsel" , "Young woman - servant" .)