A couple of dogs slim down (7)
I believe the answer is:
curtail
'slim down' is the definition.
(to curtail is to trim)
'a couple of dogs' is the wordplay.
'a couple of dogs' can mean two replacements for 'dog'.
'dog' becomes 'cur' (both can mean a despicable person).
'dog' becomes 'tail' (synonyms).
'cur'+'tail'='CURTAIL'
(Other definitions for curtail that I've seen before include "Cut short like canine appendage" , "Dock" , "Impose a restriction on" , "Reduce in extent" , "make shorter" .)