Plump for new top (3)
I believe the answer is:
opt
'plump for' is the definition.
(I've seen this before)
'new top' is the wordplay.
'new' indicates an anagram.
'top' with letters rearranged gives 'OPT'.
(Other definitions for opt that I've seen before include "Elect" , "Plump for" , "Have preference for something" , "Pick from pot" , "End or act of shutting" .)