Collar on old boy's top dog (5)
I believe the answer is:
nabob
'top dog' is the definition.
(nabob is a wealthy or powerful person)
'collar on old boy's' is the wordplay.
'collar' becomes 'nab' (both can mean to catch a person).
'on' means one lot of letters go next to another.
'old boy' becomes 'ob'.
'nab'+'ob'='NABOB'
(Other definitions for nabob that I've seen before include "he may be loaded" , "Wealthy influential person" , "Mogul governor; very rich or important person" , "Powerful man" , "Rich British merchant in India" .)