Crossword Genius

Sharp as a detectives' group (4)

Ross

I believe the answer is:

acid

I'm a little stuck... Click here to teach me more about this clue!

'sharp' is the definition.
(as in sharp-tasting)

'a detectives' group' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'detectives'' could be 'cid' (abbreviation for Criminal Investigation Department) and 'cid' is found within the answer.
No letters remain.
This explanation may well be incorrect...

'as' is the link.

Can you help me to learn more?

(Other definitions for acid that I've seen before include "Corrosive fluid" , "Substance which turns litmus red" , "Hallucinogenic drug" , "So-called drug" , "Cutting" .)

I've seen this clue in The Mirror.
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