Warning from nurse outside hospital (6)
I believe the answer is:
threat
'warning' is the definition.
(threat is a kind of warning)
'nurse outside hospital' is the wordplay.
'nurse' becomes 'treat' (I've seen this before).
'outside' means one lot of letters goes inside another.
'hospital' becomes 'h'.
'treat' enclosing 'h' is 'THREAT'.
'from' is the link.
(Other definitions for threat that I've seen before include "Statement of intent to do harm" , "Intimidatory words to exact compliance" , "Warning of intent to harm" , "Statement of harmful intent" , "The rat became a menace" .)