Bug I planted on advocate to be avenged (3,4,3,4)
I believe the answer is:
get ones own back
'to be avenged' is the definition.
(to get one's own back is to avenge oneself)
'bug i planted on advocate' is the wordplay.
'bug' becomes 'get' (to get at someone is to annoy or bug them).
'i' becomes 'one' (Roman numeral for one).
'planted' becomes 'sown' (similar in meaning).
'on' means one lot of letters go next to another.
'advocate' becomes 'back'.
'get'+'one'+'sown'+'back'='GET ONES OWN BACK'
(Other definitions for get ones own back that I've seen before include "do like The Avengers!" , "even the score?" .)