Come back about worthless dog (5)
I believe the answer is:
recur
'come back' is the definition.
(recurring is a kind of coming back)
'about worthless dog' is the wordplay.
'about' becomes 're' (regarding).
'worthless dog' becomes 'cur' (I've seen this before).
're'+'cur'='RECUR'
(Other definitions for recur that I've seen before include "Happen again (5)" , "Be repeated" , "Return to mind" , "Persist" , "come back to one's mind" .)