Natural iodine source that's supporting dog (6)
I believe the answer is:
kelpie
'natural iodine source' is the definition.
The definition suggests a singular noun which matches the answer.
'that's supporting dog' is the wordplay.
'that's' becomes 'ie'.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'dog' becomes 'kelp' (I can't explain this - if you can you should give a lot more credence to this answer).
'ie' put after 'kelp' is 'KELPIE'.
Can you help me to learn more?
(Other definitions for kelpie that I've seen before include "A Scottish water-spirit in the shape of a horse" , "A malevolent Scottish water spirit like a horse" , "Malevolent Scottish spirit in form of horse" , "Scottish water sprite -- Australian sheepdog" .)