Warning nurse about hospital (6)
I believe the answer is:
threat
'warning' is the definition.
(threat is a kind of warning)
'nurse about hospital' is the wordplay.
'nurse' becomes 'treat' (I've seen this before).
'about' indicates putting letters inside.
'hospital' becomes 'h'.
'treat' enclosing 'h' is 'THREAT'.
(Other definitions for threat that I've seen before include "An intimidating warning" , "Expression of intention to harm" , "Endangerment" , "A warning of harm to come" , "Hatter (anag)" .)