Wary load of passengers ingest drug (7)
I believe the answer is:
careful
'wary' is the definition.
(similar in meaning)
'load of passengers ingest drug' is the wordplay.
'load of passengers' becomes 'carful' (a car full of passengers).
'ingest' is an insertion indicator (inserted letters are ingested or eaten).
'drug' becomes 'e' (E is another name for ecstasy).
'carful' going around 'e' is 'CAREFUL'.
(Other definitions for careful that I've seen before include "Watch it!" , "Painstaking" , "Thrifty" , "'Prudent, cautious (7)'" , "Exercising caution" .)