Criminal British chemicals company once filling incorrect tanks in the van (7)
I believe the answer is:
illicit
'criminal' is the definition.
(I've seen this before)
'british chemicals company once filling incorrect tanks in the van' is the wordplay.
'british chemicals' becomes 'll' (I can't explain this - if you can you should give a lot more credence to this answer).
'company once' becomes 'ICI' (former company Imperial Chemical Industries).
'filling' means one lot of letters goes inside another.
'incorrect' is an anagram indicator.
'the van' indicates taking the first letters (van is the front part of something).
The first letters of 'tanks in' is 'ti'.
'll'+'ici'='llici'
'ti' anagrammed gives 'it'.
'llici' inserted into 'it' is 'ILLICIT'.
Can you help me to learn more?
(Other definitions for illicit that I've seen before include "Contrary to or forbidden by law" , "Bootleg; clandestine" , "illegitimate" , "not conforming to regulations" , "breaking the law" .)