Do your utmost and find a line of descent (6)

I believe the answer is:

strain

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'line of descent' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for strain that I've seen before include "Music" , "Tension on trains" , "making a great demand on the emotions?" , "Screen" , "Great effort - sieve" .)

I've seen this clue in the Evening Standard.

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