It’s always held back when jumpy (7)
I believe the answer is:
restive
'jumpy' is the definition.
(synonyms)
'it's always held back when' is the wordplay.
I cannot really see how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is present in the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for restive that I've seen before include "Unable to stay still" , "Unsettled" , "Fidgety, uneasy" , "Unable to keep still" , "Impatient of authority" .)