It's to let out a killer perhaps (8)
I believe the answer is:
stiletto
'a killer perhaps' is the definition.
I can't judge whether this defines the answer.
'it's to let out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'its'+'to'+'let'='itstolet'
'itstolet' is an anagram of 'STILETTO'.
Can you help me to learn more?
(Other definitions for stiletto that I've seen before include "Dagger with slender blade" , "Dagger; tapered heel" , "Type of shoe" , "part of boot?" , "Type of heel" .)