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 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' indicates an anagram.
'bath' with letters rearranged gives 'habt'.
'i' inserted 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" .)