Lacklustre youngster went back to detectives (7)

I believe the answer is:

flaccid

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

'youngster went back to detectives' is the wordplay.
'youngster' becomes 'calf' (I've seen this before).
'went back' is a reversal indicator.
'to detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'calf' reversed gives 'flac'.
'flac'+'cid'='FLACCID'

Can you help me to learn more?

(Other definitions for flaccid that I've seen before include "Drooping, soft" , "Limp, flabby" , "Hanging loose, not firm" , "Droopy" , "Slack" .)

I've seen this clue in The Daily Express.

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