Fish workers' means of effecting a run-through (9)
I believe the answer is:
pikestaff
'means of effecting a run-through' is the definition.
(you might stab or run through something with one)
'fish workers'' is the wordplay.
'fish' becomes 'pike' (pike is a kind of fish).
'workers'' becomes 'staff' (I am not sure about this - if you are sure you should believe this answer much more).
'pike'+'staff'='PIKESTAFF'
(Other definitions for pikestaff that I've seen before include "Shaft, proverbially plain" , "Mediaeval lance" , "that's very plain!" , "wooden rod?" , "Weapon on a pole" .)