Piece of ginger? Initially rare one unknown among roots? (7)
I believe the answer is:
rhizome
'piece of ginger? initially' is the definition.
I can't tell whether this definition defines the answer.
'rare one unknown among roots?' is the wordplay.
'rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'one' becomes 'i' (Roman numeral).
'unknown' becomes 'z' (algebra).
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'roots?' becomes 'home' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'z'='iz'
'iz' put within 'home' is 'hizome'.
'r'+'hizome'='RHIZOME'
Can you help me to learn more?
(Other definitions for rhizome that I've seen before include "part of plant" , "that supplies stock" , "Underground rootstock of a plan" , "Type of rootstock" , "Root" .)