A dollar for a rabbit (4)
I believe the answer is:
buck
I believe this is a double definition.
'a dollar' is the first definition.
('buck' can be a synonym of 'dollar')
'a rabbit' is the second definition.
(I have seen 'Male rabbit' mean 'buck' so perhaps 'rabbit' could also mean 'buck')
'for' is the link.
(Other definitions for buck that I've seen before include "Male rabbit, deer etc" , "Male animal - dollar" , "Male beast" , "A slangy American dollar, may be fast" , "Go against" .)