Customary line in bath design (5)
I believe the answer is:
habit
'customary line in bath design' is the definition.
I can't judge whether this definition defines the answer.
'line in bath design' is the wordplay.
'line' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'design' indicates anagramming the letters.
'bath' anagrammed gives 'habt'.
'i' going inside 'habt' is 'HABIT'.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Custom, nun's wear" , "Custom or monk's robe" , "Custom; clothing" , "Custom or established practice" , "can cause addiction" .)