Traffic force includes thousands (8)
I believe the answer is:
commerce
'traffic' is the definition.
(I've seen this before)
'force includes thousands' is the wordplay.
'force' becomes 'coerce' (synonyms).
'includes' indicates putting letters inside.
'thousands' becomes 'mm' (the Roman numeral for a thousand written twice).
'coerce' going around 'mm' is 'COMMERCE'.
(Other definitions for commerce that I've seen before include "Business dealings" , "Activity of buying and selling" , "Trading exchanges" , "The buying and selling of commodities and services" , "Trading activity" .)