Mob breaks up after sailor produces weapon (1-4)
I believe the answer is:
a-bomb
'weapon' is the definition.
(I've seen this before)
'mob breaks up after sailor' is the wordplay.
'breaks up' indicates anagramming the letters.
'after' means one lot of letters go next to another.
'sailor' becomes 'ab' (abbreviation for able seaman).
'mob' anagrammed gives 'omb'.
'omb' put after 'ab' is 'A-BOMB'.
'produces' is the link.
(Other definitions for a-bomb that I've seen before include "deterrent" , "Nuclear weapon" , "Killer" , "explosive" , "Atomic weapon" .)