Iodine oddly found in vital organ (6)
I believe the answer is:
kidney
'organ' is the definition.
(kidney is a kind of organ)
'iodine oddly found in vital' is the wordplay.
'oddly' means one should take alternating letters.
'found in' means one lot of letters goes inside another.
'vital' becomes 'key' (I've seen this before).
The alternating letters of 'iodine' are 'idn'.
'idn' put within 'key' is 'KIDNEY'.
(Other definitions for kidney that I've seen before include "Sort" , "Nephritic organ" , "class" , "temperament" , "Excreting organ" .)