Temperamental (5)
'temperamental' is the definition.
(I know that temperamental can be written as moody)
This is all the clue.
(Other definitions for moody that I've seen before include "Irritable" , "Liable to sulking" , "Down in the dumps" , "Temperamental -- capricious" , "Changeable, sullen" .)
(Other definitions for highly strung that I've seen before include "Tense and easily upset" , "Very nervous or volatile" , "Nervous, easily upset" , "Excitable - easily upset" , "like the violino piccolo" .)