The first murderer is able to take one in (4)
I believe the answer is:
cain
'the first murderer' is the definition.
(I've seen this in another clue)
'able to take one in' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'able' could be 'can' and 'can' is found within the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for cain that I've seen before include "Adam's son" , "First person born in Bible" , "Biblical killer" , "Brother of Abel" , "First murderer reported in the Bible" .)