Terribly rare twitch is unpredictable (7)
I believe the answer is:
erratic
'unpredictable' is the definition.
(synonyms)
'terribly rare twitch' is the wordplay.
'terribly' indicates an anagram.
'twitch' becomes 'tic' (tic is a kind of twitch).
'rare' with letters rearranged gives 'erra'.
'erra'+'tic'='ERRATIC'
'is' acts as a link.
(Other definitions for erratic that I've seen before include "Irregular, not dependable" , "Irregular or uneven in movement or habits" , "Not regular in pattern" , "Fluctuating" , "Unpredictable, temperamental" .)