Three can be useful in hospital (5)
I believe the answer is:
ether
'useful in hospital' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'three can be' is the wordplay.
'can be' indicates an anagram.
'three' anagrammed gives 'ETHER'.
Can you help me to learn more?
(Other definitions for ether that I've seen before include "Once widely-used anaesthetic" , "Old anaesthetic -- three (anag)" , "Highly flammable liquid once used as an anaesthetic" , "Colourless liquid used as a solvent" , "Imaginary interstellar medium" .)