Half of us had been lying, so were killed (5)
I believe the answer is:
slain
'killed' is the definition.
(slaying is a kind of killing)
'half of us had been lying so were' is the wordplay.
I cannot quite see how this works, but
'been' could be 'lain' (lying is a kind of being) and 'lain' is located in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for slain that I've seen before include "Slaughtered" , "Violently killed" , "Killed, murdered" , "Put to the sword" .)