Swear dog's evil at first (5)
I believe the answer is:
curse
'swear' is the definition.
(I know that swear can be written as curse)
'dog's evil at first' is the wordplay.
'dog's' becomes 'cur's' (both can mean a despicable person).
'at first' suggests taking the first letters.
The first letter of 'evil' is 'e'.
'curs'+'e'='CURSE'
(Other definitions for curse that I've seen before include "Utter obscenities" , "Malediction or expletive" , "Excommunication" , "Blaspheme; damn" , "Anathema" .)