Unit left in big room made a mess (9)
I believe the answer is:
imbroglio
'a mess' is the definition.
(both can mean a mess or confusion)
'unit left in big room' is the wordplay.
I cannot really understand how this works, but
'unit' could be 'mb' (Mb is an example) and 'mb' is located in the answer.
'left' could be 'l' (common abbreviation) and 'l' is found in the answer.
This explanation may well be incorrect...
'made' acts as a link.
Can you help me to learn more?
(Other definitions for imbroglio that I've seen before include "Oilrig mob (anag.)" , "massive jam" , "Confused situation results" , "Confusion" , "Dilemma" .)