Dieter's sure about a drop of valium in coffee (4)
I believe the answer is:
java
'coffee' is the definition.
(I know that coffee can be written as java)
'dieter's sure about a drop of valium' is the wordplay.
'dieter's sure' becomes 'ja' ('yes' in German - Dieter is a German name).
'about' shows that the letters should be reversed in order.
'drop of' says to take the initial letters.
The first letter of 'valium' is 'v'.
'a'+'v'='av'
'av' written backwards gives 'va'.
'ja'+'va'='JAVA'
'in' acts as a link.
(Other definitions for java that I've seen before include "Jakarta on this island" , "Indonesian island - computer programming language" , "Large island; coffee type" , "Jakarta's island" , "Coffee (slang)" .)