Occupying fast car circling round block (5)
I believe the answer is:
ingot
'block' is the definition.
(ingot is a kind of block)
'occupying fast car circling round' is the wordplay.
'occupying' becomes 'in' (I've seen this in other clues).
'fast car' becomes 'gt' (I've seen this in other clues).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'round' becomes 'o' (round can mean a round object**).
'gt' placed around 'o' is 'got'.
'in'+'got'='INGOT'
(Other definitions for ingot that I've seen before include "this must be cast" , "Form of gold or silver" , "Block of cast metal" , "Mass of metal, especially gold or silver" , "Block of, eg, gold" .)