Extra meeting with manager is up in the air (8)
I believe the answer is:
'up in the air' is the definition.
(I've seen this in another clue)
'extra meeting with' is the wordplay.
'extra' becomes 'over' (both can mean left over).
'meeting with' becomes 'head' (I can't explain this - if you can you should give a lot more credence to this answer).
'manager is' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for overhead that I've seen before include "Immediately above", "Operating cost", "Cost of upkeep of business premises, etc", "An expense in curred in maintaining property", "mostly sky-high?".)