### Clue: Criminal British chemicals company once filling incorrect tanks in the van (7)
### Answer: ILLICIT 
 ### Explanation: Definition: Criminal
 ICI(abbrev. for "Imperial Chemical Industries", once a British chemicals company) **contained in**(filling) [ ILL(incorrect/faulty, as in "ill judgement") + **1st letter of**(… in the van /in the lead) "tanks".