Previously unknown tropical juice (5)
I believe the answer is:
latex
'tropical juice' is the definition.
'latex' can be an answer for 'juice' (I have seen 'Rubbery juice ' mean 'latex' so perhaps 'juice' could also mean 'latex'). I'm unsure of the 'tropical' bit.
'previously unknown' is the wordplay.
'previously' becomes 'late' (I am not sure about this - if you are sure you should believe this answer much more).
'unknown' becomes 'x' (algebra).
'late'+'x'='LATEX'
Can you help me to learn more?
(Other definitions for latex that I've seen before include "Rubber-tree fluid" , "Milky substance produced by some trees" , "Rubbery juice" , "Exalt (anag)" , "Milky fluid of rubber tree" .)