Top dog on the phone? (4)
I believe the answer is:
peak
'top' is the definition.
(synonyms)
'dog on the phone?' is the wordplay.
'dog' becomes 'peke' (short for Pekingese).
'on the phone?' indicates a 'sounds like' (homophone) clue (I've seen 'on the phone' mean this).
'peke' sounds like 'PEAK'.
(Other definitions for peak that I've seen before include "Part of cap; mountain" , "Mountain top" , "Acme" , "High point" , "Pinnacle" .)