Buck sounds expensive (4)
I believe the answer is:
deer
'buck' is the definition.
(thesaurus)
'sounds expensive' is the wordplay.
'sounds' shows a homophone (sound like).
'expensive' becomes 'dear' (I've seen this before).
'dear' sounds like 'DEER'.
(Other definitions for deer that I've seen before include "Sledge animals for Father Christmas" , "The males have antlers" , "does for example" , "Hoofed grazing animal" , "Antlered animal" .)