How one gets into the bath is a regular thing (5)
I believe the answer is:
habit
'how one gets into the bath' is the definition.
I can't tell whether this defines the answer.
'regular thing' is the wordplay.
'regular' becomes 'hab' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'thing' becomes 'it'.
'hab'+'it'='HABIT'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "addictive practice" , "What might one have a regular tendency to" , "Addiction to a drug" , "nun gets into" , "bent" .)