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 can't justify this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'design' is an anagram indicator.
'bath' with letters rearranged gives 'habt'.
'i' going into '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" .)