A portal with small stones (6)
I believe the answer is:
agates
'stones' is the definition.
(I've seen this before)
'a portal with small' is the wordplay.
'portal' becomes 'gate' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'gate'+'s'='AGATES'
(Other definitions for agates that I've seen before include "Banded semiprecious stones" , "Hard stones with concentric bands of colour" , "marbles" , "Name for stones" , "Stones to be found in a stage" .)