An assembly of police; about a thousand get together (7)
I believe the answer is:
compile
'get together' is the definition.
(thesaurus)
'an assembly of police about a thousand' is the wordplay.
'an assembly of' indicates anagramming the letters.
'about' is an insertion indicator.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'police' with letters rearranged gives 'copile'.
'copile' enclosing 'm' is 'COMPILE'.
(Other definitions for compile that I've seen before include "Assemble (a collection)" , "Put together" , "Marshal" , "Polemic (anag.)" .)