Neat joints in fashion
I believe the answer is:
shipshape
'neat' is the definition.
('shipshape' can be a synonym of 'neat')
'joints in fashion' is the wordplay.
'joints' becomes 'hips' (hip is a kind of joint).
'in' means one lot of letters goes inside another.
'fashion' becomes 'shape' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'hips' going into 'shape' is 'SHIPSHAPE'.
Can you help me to learn more?
(Other definitions for shipshape that I've seen before include "In good order, like well-dressed sailor?" , "Neat and tidy" , "Hash pipes may be in good order - like sailor?" , "Neat and trim" , "fashionable in Bristol" .)