Strong metal (4)
I believe the answer is:
iron
'strong metal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iron that I've seen before include "1 down, symbol Fe" , "Box-shaped instrument of torture -- no meridian (anag)" , "Smooth - metal" , "Fe - golf club" , "Metallic element, symbol Fe" .)