Illegally trading criminal gang, one with external difference (13)
I believe the answer is:
contrabandist
'illegally trading' is the definition.
I can't tell whether this definition defines the answer.
'criminal gang one with external difference' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'gang' becomes 'band' (both can mean a group of people).
'one' becomes 'i' (Roman numeral).
'with external' is an insertion indicator.
'difference' becomes 'trast' (I can't explain this - if you can you should give a lot more credence to this answer).
'band'+'i'='bandi'
'bandi' put into 'trast' is 'trabandist'.
'con'+'trabandist'='CONTRABANDIST'
Can you help me to learn more?
(Other definitions for contrabandist that I've seen before include "Illegal trader" , "Neglecter of duty" , "Runner" , "Smuggler" .)