The important part - but not of an orange (4)
I believe the answer is:
pith
'the important part' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pith that I've seen before include "Tissue beneath citrus fruit rind" , "hats may be made of this" , "Core - essence" , "Spongy tissue like that lining an orange skin" , "Citrus rind lining" .)