Blacksmiths' blocks (6)
I believe the answer is:
anvils
'blacksmiths' blocks' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for anvils that I've seen before include "Heavy blocks for the blacksmith" , "Blocks for a blacksmith to work on" , "Heavy blocks for blacksmiths" , "Iron blocks for blacksmiths" , "Iron blocks for hammering hot metal" .)