Tunnels once sheltered famous commander (6)

I believe the answer is:

nelson

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'sheltered famous commander' is the definition.
The definition suggests a singular noun which matches the answer.

'tunnels once' is the wordplay.
'NELSON' is hidden within the letters.
However, I am unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for nelson that I've seen before include "Admiral called 5" , "Naval hero who died at Trafalgar" , "Maritime chief" , "'Horatio ..., English admiral (6)'" , "Admiral (who had the touch)" .)

I've seen this clue in The Irish Times.

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