Twice be right over start of invasive tropical disease (4-4)
I believe the answer is:
beri-beri
'tropical disease' is the definition.
(I've seen this before)
'twice be right over start of invasive' is the wordplay.
'twice' shows that letters should be duplicated.
'right' indicates one should take the final letters (I've seen 'right of' mean this (letter on the rightmost end of the word)).
'start of' indicates taking the first letters.
The last letter of 'over' is 'r'.
The initial letter of 'invasive' is 'i'.
'be'+'r'+'i'='beri'
'beri' duplicated is 'BERI-BERI'.
(Another definition for beri-beri that I've seen is " tropical disease".)