Three could put you to sleep! (5)
I believe the answer is:
ether
'sleep' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'three could put you' is the wordplay.
'three' can be anagrammed to 'ETHER'.
However, I am unsure how the anagram is indicated.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for ether that I've seen before include "Anaesthetic; clear sky (poetic)" , "Chemical solvent" , "Imaginary interstellar medium" , "The gas is there, oddly" , "It could knock you out" .)