Animal opening hospital (5)
I believe the answer is:
'animal' is the definition.
(sloth is a kind of animal)
'opening hospital' is the wordplay.
'opening' becomes 'slot' (slot is a kind of opening).
'hospital' becomes 'H' (abbreviation for hospital).
(Other definitions for sloth that I've seen before include "one of a deadly group" , "Deadly offence" , "reluctance to work" , "Arboreal creature" , "Laziness; slow mammal" .)