I I had turned into I I (6)
I believe the answer is:
iodine
'i' is the definition.
('iodine' can be a synonym of 'i')
'i had turned into i i' is the wordplay.
'i had' becomes 'id' (I'd).
'turned' shows that the letters should be reversed in order.
'into' means one lot of letters goes inside another.
'i' becomes 'one' ('one' can be a synonym of 'i').
'id' back-to-front is 'di'.
'i'+'one'='ione'
'di' put into 'ione' is 'IODINE'.
(Other definitions for iodine that I've seen before include "Element used in medicine, photography and dyes" , "Tincture for cuts" , "Element, atomic number 53" , "Black/violet element" , "Element forming black crystals or violet vapour" .)