Main threat stole lead from trader with uranium (1-4)
I believe the answer is:
Here is my best explanation:
'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').
'boat' after 'u' is 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "German wartime submarine", "Sub", "Sea vessel", "German submarine of WW2", "German World War One or World War Two submarine", "Take pool exercise", "Submersible warship armed with torpedoes", "About (anag)", "German WW 2 sub" and "Submersible warship of WWII".)