Informal plumbing fixture provides seat for monarch (6)
I believe the answer is:
throne
This clue could be a double definition.
'informal plumbing fixture' is the first definition.
'throne' can be an answer for 'fixture' (throne is a kind of fixture). I'm not sure about the 'informal plumbing' bit.
'seat for monarch' is the second definition.
'throne' can be an answer for 'seat' (I've seen this before). I'm not certain of the 'for monarch' bit.
'provides' acts as a link.
Can you help me to learn more?
(Other definitions for throne that I've seen before include "Seat of royalty" , "Bishop's chair" , "Seat of monarch" , "Sovereign's seat" , "Sovereign's chair" .)