Blacksmiths' blocks needed about five new nails (6)
I believe the answer is:
anvils
'blacksmiths' blocks' is the definition.
(I've seen this in another clue)
'about five new nails' is the wordplay.
'about' is an insertion indicator.
'five' becomes 'V' (Roman numeral).
'new' is an anagram indicator.
'nails' with letters rearranged gives 'anils'.
'v' placed inside 'anils' is 'ANVILS'.
'needed' acts as a link.
(Other definitions for anvils that I've seen before include "Heavy blocks for blacksmiths" , "Heavy blocks for the blacksmith" , "Iron blocks for hammering hot metal" , "Blocks for a blacksmith to work on" , "Iron blocks for blacksmiths" .)