Jittery as always; it's through being upset (7)
I believe the answer is:
restive
'jittery' is the definition.
(I know that jittery can be written as restive)
'it's through being upset' is the wordplay.
'through' means one lot of letters goes inside another (inserted letters go through the word).
'being' becomes 'ever' (I can't explain this - if you can you should believe this answer much more).
'upset' says the letters should be written in reverse.
'its' put inside 'ever' is 'evitser'.
'evitser' back-to-front is 'RESTIVE'.
'as always' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for restive that I've seen before include "Unsettled" , "It veers around, resisting control" , "Unable to stay still" , "Fidgety, uneasy" , "Unable to keep still" .)