Sharp detectives in top case's opening (6)
I believe the answer is:
acidic
'sharp' is the definition.
(I've seen this before)
'detectives in top case's opening' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in' indicates putting letters inside.
'top' becomes 'ai' (resembles A1, term for excellent).
'opening' says to take the initial letters.
The first letter of 'cases' is 'c'.
'cid' inserted within 'ai' is 'acidi'.
'acidi'+'c'='ACIDIC'
(Other definitions for acidic that I've seen before include "Like vinegar" , "Tart, sour" , "Vinegary" , "Sharp-tasting, sour" , "scathing?" .)