Rocky tower in Stornoway (3)

I believe the answer is:

tor

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'rocky tower' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'in stornoway' is the wordplay.
'in' says the answer is hidden in the clue.
'TOR' can be found hidden inside 'stornoway'.

Can you help me to learn more?

(Other definitions for tor that I've seen before include "Rot on rocky peak" , "fell" , "Rot on a hill in Devon" , "rise" , "Pillar of rock" .)

I've seen this clue in The Sun.

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