In high definition, I study the unknown (6)
I believe the answer is:
'the unknown' is the definition.
'in high definition i study' is the wordplay.
'in' means one lot of letters goes inside another.
'high definition' becomes 'hd' (this could be a standard abbreviation which I don't know about).
'study' becomes 'den' (term for a study or office).
'hd' placed around 'i' is 'hid'.
Can you help me to learn more?
(Other definitions for hidden that I've seen before include "Deliberately placed out of sight and hard to find" , "Obscure" , "Covert, obscured" , "Concealed, recondite" , "Consealed or secret" .)