Bob not half a scoundrel (3)
I believe the answer is:
cur
'a scoundrel' is the definition.
(cur can mean a despicable person)
'bob not half' is the wordplay.
'bob' becomes 'curtsy' (synonyms).
'not half' means to take half the letters.
'curtsy' cut in half is 'CUR'.
(Other definitions for cur that I've seen before include "Vicious dog" , "Surly dog (or person)" , "Mongrel - coward" , "Mongrel - cowardly person" , "Poor dog" .)