Threaten, harass (6)
I believe the answer is:
menace
This clue is a double definition.
'threaten' is the first definition.
(synonyms)
'harass' is the second definition.
(thesaurus)
(Other definitions for menace that I've seen before include "ominous atmosphere" , "Threaten with harm or danger" , "Intimidate" , "Threat, danger" , "Act in a threatening manner" .)