Bath users may catch it (3,4,3,5)
I believe the answer is:
get into hot water
'bath users' is the definition.
I know nothing about this answer so I can't judge whether this works.
'catch it' is the wordplay.
I cannot really see how this works, but
'catch' could be 'get' (synonyms) and 'get' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
This explanation may well be incorrect...
'may' is the link.
Can you help me to learn more?
(Other definitions for get into hot water that I've seen before include "as you do, in a jacuzzi" , "Find trouble" .)