Felt secure in county town (7)

I believe the answer is:

matlock

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'county town' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?

'felt secure' is the wordplay.
'felt' becomes 'mat'.
'secure' becomes 'lock' (locking is a kind of securing).
'mat'+'lock'='MATLOCK'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for matlock that I've seen before include "administrative centre" , "Derbyshire town" .)

I've seen this clue in The Independent.

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