Copper on beat missing half of block (4)
I believe the answer is:
cube
'block' is the definition.
(synonyms)
'copper on beat missing half' is the wordplay.
'copper' becomes 'Cu' (chemical symbol).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'missing half' indicates taking half.
'beat' halved is 'be'.
'cu'+'be'='CUBE'
'of' acts as a link.
(Other definitions for cube that I've seen before include "Three-dimensional shape with six sides" , "Lump of sugar" , "Times itself twice" , "Square-sided solid" , "Hexagonal ice" .)