I get into the bath perhaps as a matter of routine (5)
I believe the answer is:
habit
'routine' is the definition.
(I've seen this before)
'i get into the bath perhaps' is the wordplay.
'get into' means one lot of letters goes inside another.
'perhaps' indicates an anagram.
'bath' with letters rearranged gives 'habt'.
'i' put into 'habt' is 'HABIT'.
'as a matter of' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "What might one have a regular tendency to" , "Usual practice; garment" , "bent" , "Custom; riding dress" , "It's second nature" .)