Inter twice, they say, with thiamine deficiency (4-4)
I believe the answer is:
beri-beri
'they say with thiamine deficiency' is the definition.
I know nothing about this answer so I can't judge whether this works.
'inter twice' is the wordplay.
'inter' becomes 'beri' (I can't justify this - if you can you should believe this answer much more).
'twice' shows that letters should be duplicated.
'beri' doubled is 'BERI-BERI'.
Can you help me to learn more?
(Another definition for beri-beri that I've seen is " tropical disease".)