Three may 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 can't understand how they can define each other.
'three may put you' is the wordplay.
'three' anagrams to 'ETHER'.
However, I'm not sure 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 "Medium supposedly" , "Anaesthetic is, strangely, there" , "Light liquid used as anaesthetic" , "Chemical solvent" , "Type of chemical" .)