Drug smuggler smuggles too many drugs in self-contained unit (6)
I believe the answer is:
'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 "Element of course etc." , "Detachable compartment of a spacecraft" , "Self-contained unit, of study perhaps" , "Independent unit in furniture, building etc" , "Detachable spacecraft unit" .)