Maybe past getting jittery (5)
I believe the answer is:
tense
I believe this is a double definition.
'maybe past' is the first definition.
(I've seen this before)
'jittery' is the second definition.
(similar in meaning)
'getting' acts as a link.
(Other definitions for tense that I've seen before include "On edge" , "Anxious, nervous" , "(Of muscle) tight" , "past possibly" , "edgy" .)