Whittled down slab, a custom-made counter (6)
I believe the answer is:
abacus
'counter' is the definition.
(device for counting with)
'whittled down slab a custom-made' is the wordplay.
'ABACUS' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for abacus that I've seen before include "Counting machine" , "An old counting frame with beads" , "It will support the architrave" , "Beaded counting frame" , "It might calculate" .)