Getting a nervous twitch when hungry (8)
I believe the answer is:
ravenous
'hungry' is the definition.
(similar in meaning)
'getting a nervous twitch' is the wordplay.
'getting' means one lot of letters goes inside another.
'twitch' indicates anagramming the letters (I've seen 'twitching' mean this (letters twitch or move into a new order)).
'nervous' with letters rearranged gives 'rvenous'.
'a' going into 'rvenous' is 'RAVENOUS'.
'when' acts as a link.
(Other definitions for ravenous that I've seen before include "Intensely hungry" , "Wolfish" , "Really hungry, ready to devour" , "Extremely hungry" , "Voracious" .)