A portal's made of stones (6)
I believe the answer is:
agates
'stones' is the definition.
(I've seen this before)
'a portal's made' is the wordplay.
'portal' becomes 'gate' (I've seen this before).
'made' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'gate'+'s'='AGATES'
'of' is the link.
Can you help me to learn more?
(Other definitions for agates that I've seen before include "Stones to be found in a stage" , "marbles" , "Banded semiprecious stones" , "Chalcedonies" , "Name for stones" .)