Threaten males with one (6)
I believe the answer is:
menace
'threaten' is the definition.
(I know that threaten can be written as menace)
'males with one' is the wordplay.
'males' becomes 'men' (man is a kind of male).
'with' says to put letters next to each other.
'one' becomes 'ace' (ace has a value of one in some card games).
'men'+'ace'='MENACE'
(Other definitions for menace that I've seen before include "Threat, danger" , "ominous atmosphere" , "A threatening quality" , "Serious threat" , "Threat or severe nuisance" .)