Still not very upset (5)
I believe the answer is:
inert
'still' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for inert that I've seen before include "(Chem.) not reactive" , "Nothing doing" , "'Not moving, lifeless (5)'" , "Nitre (anag.)" , "Sluggish - unresponsive" .)