Shabby commercial block held by small number (10)
I believe the answer is:
threadbare
'shabby' is the definition.
(I've seen this before)
'commercial block held by small number' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'block' becomes 'bar' (block can mean to ban or bar).
'held by' indicates putting letters inside.
'small number' becomes 'three' (three is a small number).
'ad'+'bar'='adbar'
'adbar' placed within 'three' is 'THREADBARE'.
(Other definitions for threadbare that I've seen before include "Shabby" , "Ragged" , "Well-worn" , "Hackneyed" , "the worse for wear" .)