Three could put you out (5)
I believe the answer is:
ether
'put you out' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'three' is the wordplay.
'three' anagrams to 'ETHER'.
But, I am unsure how the anagram is indicated.
'could' is the link.
Can you help me to learn more?
(Other definitions for ether that I've seen before include "Some were put out by this" , "Type of chemical" , "Solvent; clear sky (poet.)" , "Volatile solvent" , "Imaginary interstellar medium" .)