Inscrutable quartet in deadlock (9)

I believe the answer is:

impassive

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'inscrutable' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

'quartet in deadlock' is the wordplay.
'quartet' becomes 'IV' (Roman numerals for four).
'in' is an insertion indicator.
'deadlock' becomes 'impasse' (synonyms).
'iv' put into 'impasse' is 'IMPASSIVE'.

Can you help me to learn more?

(Other definitions for impassive that I've seen before include "Undemonstrative" , "unmoved" , "Phlegmatic" , "unfeeling" , "Not showing emotions in face" .)

I've seen this clue in The Sun.

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