It’s for water but contains oil (6)
I believe the answer is:
boiler
'it's' is the definition.
I can't judge whether this defines the answer.
'water but contains oil' is the wordplay.
'water but' becomes 'ber' (I can't explain this - if you can you should believe this answer much more).
'contains' indicates putting letters inside.
'ber' placed around 'oil' is 'BOILER'.
'for' is the link.
Can you help me to learn more?
(Other definitions for boiler that I've seen before include "tough old bird" , "Water heater" , "Steam-producer" , "Water-heater" , "Container for heating water or a tough fowl" .)