A tense "No improvement is possible for me" (9)

I believe the answer is:

imperfect

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'a tense' is the definition.
(the imperfect tense is a grammatical tense)

I cannot understand how the rest of the clue works.

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(Other definitions for imperfect that I've seen before include "Deficient" , "Faulty; tense" , "Flawed, defective" , "being immature" , "Describing continuous actions" .)

I've seen this clue in the Evening Standard.

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