Forbidden content word of first ten (8)
I believe the answer is:
verboten
'forbidden' is the definition.
(thesaurus)
'content word of first ten' is the wordplay.
'content word' becomes 'verb' (verb is a kind of content word).
'first' says to take the initial letters.
The first letter of 'of' is 'o'.
'verb'+'o'+'ten'='VERBOTEN'
(Other definitions for verboten that I've seen before include "Forbidden to the Germans" , "Not sanctioned" , "Forbidden (German)" , "not allowed in German" , "Strictly not allowed" .)