Nurse admitted to offensive belief (5)
I believe the answer is:
tenet
'belief' is the definition.
(synonyms)
'nurse admitted to offensive' is the wordplay.
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'admitted to' means one lot of letters goes inside another.
'offensive' becomes 'tet' (Tet Offensive, Vietnam War military campaign).
'en' inserted into 'tet' is 'TENET'.
(Other definitions for tenet that I've seen before include "Teaching" , "Palindromic principle or belief" , "Canon" , "Belief held as true" , "Doctrine held to be true" .)