Threaten nurse in club (6)
I believe the answer is:
menace
'threaten' is the definition.
(synonyms)
'nurse in club' is the wordplay.
'nurse' becomes 'en' (abbreviation for enrolled nurse).
'in' is an insertion indicator.
'club' becomes 'mace' (I've seen this before).
'en' going inside 'mace' is 'MENACE'.
(Other definitions for menace that I've seen before include "Act in a threatening manner" , "Intimidation" , "Harmful person" , "Threaten with harm or danger" , "Threat or severe nuisance" .)