Lacking information, one or the other might knock you out (5)
I believe the answer is:
ether
'might knock you out' is the definition.
(ether is an anaesthetic so might knock you out)
'lacking information one or the other' is the wordplay.
'lacking' indicates named letters should be taken away.
'information' becomes 'i' (common abbreviation for information).
'one or the other' becomes 'either' (I've seen this before).
'either' with 'i' taken away is 'ETHER'.
(Other definitions for ether that I've seen before include "Anaesthetic is, strangely, there" , "The gas is there, oddly" , "Colourless liquid used as a solvent" , "An early anaesthetic" , "Light liquid used as anaesthetic" .)