Servant crazy to get out of line (4)
I believe the answer is:
maid
'servant' is the definition.
(maid is a type of servant)
'crazy to get out of line' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'to get' indicates putting letters inside.
'out of line' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'mad' going around 'i' is 'MAID'.
Can you help me to learn more?
(Other definitions for maid that I've seen before include "One in service" , "Female servant - young girl" , "- - -of-all-work" , "Damsel" , "Young woman - servant" .)