Assistant pitman is one who makes the tunnels (10)

I believe the answer is:

underminer

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'one who makes the tunnels' is the definition.
The definition suggests a singular noun which matches the answer.

'assistant pitman' is the wordplay.
'assistant' becomes 'under' (I can't justify this - if you can you should believe this answer much more).
'pitman' becomes 'miner' (I've seen this before).
'under'+'miner'='UNDERMINER'

'is' is the link.

Can you help me to learn more?

(Another definition for underminer that I've seen is " Sapper".)

I've seen this clue in The Irish News.

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