Words very old police force eschews on street (10)
I believe the answer is:
vocabulary
'words' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for vocabulary that I've seen before include "Knowledge of words" , "List of words" , "Word list" , "Words an individual knows" , "Range of words" .)