Vote against eating toast crusts? That's smart! (5)
I believe the answer is:
natty
'smart' is the definition.
(I've seen this before)
'vote against eating toast crusts?' is the wordplay.
'vote against' becomes 'nay' (I've seen this in another clue).
'eating' is an insertion indicator.
'crusts?' suggests removing the centre (I've seen 'crust' mean this (the outside or crust of the word is used)).
'toast' with its centre taken out is 'tt'.
'nay' placed around 'tt' is 'NATTY'.
'that's' is the link.
(Other definitions for natty that I've seen before include "Smartly dressed" , "Stylishly dressed" , "Stylish" , "Informally neat and well-dressed" , "Smart, spruce" .)