Drug smuggler smuggles too many drugs in self-contained unit (6)
I believe the answer is:
Here is my best explanation:
'self-contained unit' is the definition.
'drug smuggler smuggles too many drugs' is the wordplay.
'drug smuggler' becomes 'mule' (I've seen this before).
'smuggles' means one lot of letters goes inside another.
'too many drugs' becomes 'od' (abbreviation for overdose).
'mule' placed around 'od' is 'MODULE'.
'in' is the link.
(Other definitions for module that I've seen before include "Detachable spacecraft unit", "Self-contained unit, of study perhaps", "Complete part (of spacecraft or course)", "Detachable sub-unit", "Short study course", "Detachable compartment of a spacecraft", "Element of course etc.", "Standardised part in furniture - or spacecraft", "Independent unit in furniture, building etc" and "Unit of a study course".)