Hospital not quite secure, one admitted (6)
I believe the answer is:
clinic
'hospital' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for clinic that I've seen before include "Place for specialised treatment or advice" , "medical department" , "Healthcare facility" , "Hospital department" , "Place offering treatment" .)