Drug source keeps English detectives up (8)
I believe the answer is:
medicine
'drug' is the definition.
(I know that drug can be written as medicine)
'source keeps english detectives up' is the wordplay.
'source' becomes 'mine' (a mine is a source of something).
'keeps' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'cid' written backwards gives 'dic'.
'e'+'dic'='edic'
'mine' enclosing 'edic' is 'MEDICINE'.
(Other definitions for medicine that I've seen before include "Laughter is said to be the best one" , "Drug for treatment" , "Curative treatment" , "Profession of a physician" , "The practice of diagnosis and treatment of disease" .)