Some guys have power; start engendering a threat (6)
I believe the answer is:
menace
'a threat' is the definition.
(I know that threat can be written as menace)
'some guys have power start engendering' is the wordplay.
'some guys' becomes 'men' (guy can mean a man).
'have' means one lot of letters go next to another.
'power' becomes 'ac' (AC is an example).
'start' suggests taking the first letters.
The initial letter of 'engendering' is 'e'.
'men'+'ac'+'e'='MENACE'
(Other definitions for menace that I've seen before include "Threat; threaten" , "Threat or severe nuisance" , "Serious threat" , "ominous atmosphere" , "A threatening quality" .)