A portal of rock (5)

I believe the answer is:

agate

'rock' is the definition.

'a portal' is the wordplay.
'portal' becomes 'gate' (I've seen this in another clue).
'a'+'gate'='AGATE'

'of' acts as a link.

(Other definitions for agate that I've seen before include "Banded ornamental stone" , "Variety of chalcedony" , "Semi-precious quartz" , "Onyx" , "Decorative stone" .)

I've seen this clue in The Mirror.

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