Sharp as a detectives' group (4)
I believe the answer is:
acid
'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" .)