Make matters worse? One left out most important thing! (7)
I believe the answer is:
capital
'most important thing' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'make matters worse? one left' is the wordplay.
'make matters worse?' becomes 'capita' (I can't explain this - if you can you should believe this answer much more).
'one left' becomes 'L' (common abbreviation).
'capita'+'l'='CAPITAL'
'out' is the link.
Can you help me to learn more?
(Other definitions for capital that I've seen before include "Money saved or invested - seat of government" , "Money available to produce interest" , "Metropolis" , "Major city - investment - excellent" , "city such as London" .)