What The Killers are capable of is largely mistaken for Fairground Attraction
I believe the answer is:
shooting gallery
'what the killers are capable of' is the definition.
I can't judge whether this definition defines the answer.
'largely mistaken for fairground attraction' is the wordplay.
'mistaken' is an anagram indicator.
'for' says to put letters next to each other (I've seen this in other clues).
'fairground attraction' becomes 'shooting' (I can't justify this - if you can you should give a lot more credence to this answer).
'largely' is an anagram of 'gallery'.
'gallery' after 'shooting' is 'SHOOTING-GALLERY'.
'is' is the link.
Can you help me to learn more?
(Other definitions for shooting gallery that I've seen before include "where discharge follows training" , "A fair range" , "Where one may be on target" , "Where one might get high" , "target practice here" .)