Tropical tree needs roughly twice what we need to breathe (5)
I believe the answer is:
cacao
'tropical tree' is the definition.
(I've seen this before)
'roughly twice what we need to breathe' is the wordplay.
I cannot quite understand how this works, but
'roughly' could be 'ca' (abbreviation for circa) and 'ca' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'needs' acts as a link.
Can you help me to learn more?
(Other definitions for cacao that I've seen before include "Tree supplying chocolate" , "Tree from which chocolate is made" , "Tropical tree, the source of chocolate" , "Plant" , "Tree for cocoa bean" .)