Allow in western criminal - an informer (10)
I believe the answer is:
newsletter
'an informer' is the definition.
(something which informs)
'allow in western criminal' is the wordplay.
'allow' becomes 'let' ('let' can be a synonym of 'allow').
'in' is an insertion indicator.
'criminal' is an anagram indicator.
'western' anagrammed gives 'newster'.
'let' placed inside 'newster' is 'NEWSLETTER'.
(Other definitions for newsletter that I've seen before include "Regularly-issued bulletin" , "Society report" , "Sheet of information" , "Regular bulletin" , "Periodical bulletin" .)