Doctor has tips on using medicine (4)
I believe the answer is:
drug
'medicine' is the definition.
(I know that medicine can be written as drug)
'doctor has tips on using' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'has' means one lot of letters go next to another.
'tips on' says to hollow out the word (remove centre letters) (only the tips of the word).
'using' with its middle taken out is 'ug'.
'dr'+'ug'='DRUG'
(Other definitions for drug that I've seen before include "Perhaps grass" , "Substance used in medicine, often abused" , "Tranquillize" , "Medical or narcotic substance" , "Maybe addictive substance" .)