Stupid and having no prospect of success as a sleuth (8)

I believe the answer is:

clueless

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'stupid' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for clueless that I've seen before include "Stupid (so unable to do crossword?)" , "how I started this puzzle!" , "Totally uninformed about what's going on" , "Lacking knowledge and ability (informal)" , "Lacking in ideas" .)

I've seen this clue in the Evening Standard.

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