I say God's within (6)
I believe the answer is:
iodine
'i' is the definition.
(I is the chemical symbol for iodine)
'say god's within' is the wordplay.
'say' becomes 'ie' (I can't justify this - if you can you should give a lot more credence to this answer).
'god' becomes 'odin' (Norse god).
'within' means one lot of letters goes inside another.
'ie' placed around 'odin' is 'IODINE'.
Can you help me to learn more?
(Other definitions for iodine that I've seen before include "Solution applied to wounds as an antiseptic" , "Mother-of-pearl" , "Antiseptic found in seaweed" , "for therapeutic purposes" , "Black/violet element" .)