A portal of quartz (5)
I believe the answer is:
agate
'quartz' is the definition.
(agate is a kind of quartz)
'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 "Type of quartz" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "Ornamental form of quartz" , "Variety of chalcedony" , "Banded ornamental stone" .)