Cool chaps could be killers in Brooklyn (6)
I believe the answer is:
icemen
'killers in brooklyn' is the definition.
The definition suggests a plural noun which matches the answer.
'cool chaps' is the wordplay.
'cool' becomes 'ice' (icing is a kind of cooling).
'chaps' becomes 'men' (chap means a man).
'ice'+'men'='ICEMEN'
'could be' is the link.
Can you help me to learn more?
(Other definitions for icemen that I've seen before include "They deliver blocks" , "archaeological discoveries?" , "People who deliver frozen water" .)