Container to care about; it could be a killer (7)
I believe the answer is:
carbine
'be a killer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'container to care about it' is the wordplay.
'container' becomes 'bin' (bin is a type of container).
'about it' means one lot of letters goes inside another (some letters go around or about others).
'bin' inserted into 'care' is 'CARBINE'.
'could' acts as a link.
Can you help me to learn more?
(Other definitions for carbine that I've seen before include "Light automatic rifle -- nice bra (anag)" , "A short light automatic rifle" .)