Main threat stole lead from trader with uranium (1-4)
I believe the answer is:
u-boat
'main threat' is the definition.
(main can mean open sea)
'stole lead from trader with uranium' is the wordplay.
'stole' becomes 'boa' (I've seen this before).
'lead from trader' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'with' says to put letters next to each other.
'uranium' becomes 'u' ('U' can be a synonym of 'uranium').
'boa'+'t'='boat'
'boat' after 'u' is 'U-BOAT'.
Can you help me to learn more?
(Other definitions for u-boat that I've seen before include "German submarine of WW2" , "German WW 2 sub" , "German World War One or World War Two submarine" , "German wartime submarine" , "Take pool exercise" .)