Top dog, reportedly (4)
I believe the answer is:
peak
'top' is the definition.
('peak' can be a synonym of 'top')
'dog reportedly' is the wordplay.
'dog' becomes 'peke' (Peke is an example).
'reportedly' shows a homophone (sound like).
'peke' is a homophone of 'PEAK'.
(Other definitions for peak that I've seen before include "Acme" , "High point" , "Pointed mountain" , "Pinnacle" , "Apex" .)