One’s in the bath, perhaps, as a weekly thing (5)
I believe the answer is:
habit
'a weekly thing' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'one's in the bath perhaps' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in the' is an insertion indicator.
'perhaps' indicates anagramming the letters.
'bath' with letters rearranged gives 'habt'.
'i' going inside 'habt' is 'HABIT'.
'as' is the link.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Routine - clothing" , "Acquired pattern of behaviour" , "can cause addiction" , "Settled tendency" , "Custom, nun's wear" .)