Be certain to keep dog restrained (6)
I believe the answer is:
secure
'be certain' is the definition.
(I've seen this in another clue)
'to keep dog restrained' is the wordplay.
'to keep' indicates putting letters inside.
'dog' becomes 'cur' (both can mean a despicable person).
'restrained' becomes 'see' (I can't explain this - if you can you should give a lot more credence to this answer).
'cur' put inside 'see' is 'SECURE'.
Can you help me to learn more?
(Other definitions for secure that I've seen before include "Safe against attack or well fastened" , "Safe; fixed" , "properly restrained" , "land" , "Safe and unthreatened" .)