On time to leave messy plates in sink again (7)
I believe the answer is:
relapse
'sink again' is the definition.
('lapse' can be a synonym for 'sink' so perhaps 'relapse' is OK for 'sink again')
'on time to leave' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'time to leave' becomes 'lapse' (I can't explain this - if you can you should believe this answer much more).
're'+'lapse'='RELAPSE'
'messy plates in' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for relapse that I've seen before include "A return to illness" , "Failure to maintain a higher state" , "Downturn in health (after improvement)" , "Get into bad habits again" , "Deterioration in health" .)