A portal of stone (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'a portal' is the wordplay.
'portal' becomes 'gate' (I've seen this before).
'a'+'gate'='AGATE'
'of' acts as a link.
(Other definitions for agate that I've seen before include "Quartz with bands of colour, used in jewellery" , "Type of quartz" , "Precious stone" , "Old drama critic" , "Semi-transparent stone" .)