Boxes lighter than usual with drug in short supply (6)
I believe the answer is:
sparse
'in short supply' is the definition.
(sparse can mean meagre or minimal)
'boxes lighter than usual with drug' is the wordplay.
I cannot quite see how this works, but
'boxes' could be 'spars' (sparring is a kind of boxing) and 'spars' is found within the answer.
'drug' could be 'e' (E is another name for ecstasy) and 'e' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sparse that I've seen before include "Thin on the ground, scarce" , "Thinly populated" , "Spares, being thin on the ground" , "not packed together" , "Not much" .)