Firm notice about dog (6)
I believe the answer is:
secure
'firm' is the definition.
(similar in meaning)
'notice about dog' is the wordplay.
'notice' becomes 'see' (notice can mean to spot or see).
'about' means one lot of letters goes inside another.
'dog' becomes 'cur' (both can mean a despicable person).
'see' enclosing 'cur' is 'SECURE'.
(Other definitions for secure that I've seen before include "Firmly in place - free from danger" , "'Safe, confident (6)'" , "Safe; make fast" , "Free from danger or risk" , "Free from danger and financially sound" .)