The miserable dog is insecure (3)
I believe the answer is:
cur
I believe this clue is a double definition.
'the miserable dog' is the first definition.
(I've seen this before)
'insecure' is the second definition.
(in the letters 'seCURe')
'is' is the link.
(Other definitions for cur that I've seen before include "Surly dog (or person)" , "Despicable person" , "Mongrel - cowardly person" , "Worthless or bad-tempered dog" , "Worthless mongrel" .)