Sharp as a lot of sleuths (4)
I believe the answer is:
acid
'sharp' is the definition.
(I've seen this before)
'as a lot of sleuths' is the wordplay.
'a lot of' means to remove the last letter.
'sleuths' becomes 'cid' (I am not sure about this - if you are sure you should believe this answer much more).
'as' with its final letter taken away is 'a'.
'a'+'cid'='ACID'
Can you help me to learn more?
(Other definitions for acid that I've seen before include "Type of chemical" , "Substance with pH of less than 7" , "Sour - sharp" , "scathing" , "Alkali's opposite" .)