Swear about terrible ruse (5)
I believe the answer is:
curse
'swear' is the definition.
(I know that swear can be written as curse)
'about terrible ruse' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'terrible' indicates anagramming the letters.
'ruse' is an anagram of 'urse'.
'c'+'urse'='CURSE'
(Other definitions for curse that I've seen before include "Excommunication" , "Utter obscenities" , "Profanity" , "Invoke evil upon" , "Blaspheme; damn" .)