Beware occupants of packed vehicle carrying drug (7)
I believe the answer is:
careful
'beware occupants of packed' is the definition.
I can't tell whether this definition defines the answer.
'vehicle carrying drug' is the wordplay.
I cannot really see how this works, but
'vehicle' could be 'car' (car is a kind of vehicle) and 'car' is found within the answer.
'drug' could be 'e' (E is another name for ecstasy) and 'e' is located in the answer.
The remaining letters 'ful' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for careful that I've seen before include "'Prudent, cautious (7)'" , "Guarded" , "Thrifty" , "Exercising caution" , "Watch it!" .)