Old cat is tough, having a large family (9)

I believe the answer is:

grimalkin

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'old cat' is the definition.
'grimalkin' can be an answer for 'cat' (thesaurus). I am not sure about the 'old' bit.

'tough having a large family' is the wordplay.
'tough' becomes 'grim' (I've seen this in another clue).
'having' says to put letters next to each other.
'large' becomes 'L' (eg in clothes sizes).
'family' becomes 'kin' (synonyms).
'grim'+'a'+'l'+'kin'='GRIMALKIN'

'is' is the link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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