Block an uncompleted foul (5)
I believe the answer is:
anvil
'block' is the definition.
(anvil is a kind of block)
'an uncompleted foul' is the wordplay.
'uncompleted' means to remove the last letter.
'foul' becomes 'vile' ('vile' can be a synonym of 'foul').
'vile' with its final letter removed is 'vil'.
'an'+'vil'='ANVIL'
(Other definitions for anvil that I've seen before include "Block on which metals are hammered" , "Verdi chorus from Il Trovatore" , "Block used in forge" , "Forging equipment" , "Smith's heavy iron block" .)