No substance protects unaccompanied medic (10)

I believe the answer is:

nosologist

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'medic' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I don't see?

'no substance protects unaccompanied' is the wordplay.
'substance' becomes 'gist' (synonyms).
'protects' indicates putting letters inside (outer letters protect inside letters).
'unaccompanied' becomes 'solo' (I've seen this in another clue).
'no'+'gist'='nogist'
'nogist' enclosing 'solo' is 'NOSOLOGIST'.

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