A duck fed very hard metal block (5)
I believe the answer is:
anvil
'hard metal block' is the definition.
(an anvil is a block of metal)
'a duck fed very' is the wordplay.
'duck' becomes 'nil' (both can mean zero).
'fed' means one lot of letters goes inside another (one word is 'fed' to another).
'very' becomes 'v' (abbreviation).
'nil' placed around 'v' is 'nvil'.
'a'+'nvil'='ANVIL'
(Other definitions for anvil that I've seen before include "Heavy block on which to hammer metal" , "Iron block; incus" , "Block on which metal can be hammered" , "Block for hammering" , "Metalworker's block" .)