1 across
Extremely
boring
oldster
is
young
after
treatment
(4-10)
Extremely boring oldster is young after treatment (4-10)
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The definition is at the beginning of the clue.
How the clue works
Right, so Extremely boring is the definition — that's your tediously dull SOUL-DESTROYING. The rest, "oldster is young", is an anagram (the "after treatment" tips you off that the letters need rearranging), and shuffled together they spell out the whole answer.