Dessert's up: at last spoon out and get a move on (6)
I believe the answer is:
'get a move on' is the definition.
(command to tell a horse to go faster)
'dessert's up at last spoon out' is the wordplay.
'dessert' becomes 'pud' (short for pudding).
'up' is a reversal indicator (in down clue: letters go upwards).
'at' means one lot of letters go next to another (I've seen this in other clues).
'last spoon out' becomes 'gid' (I can't explain this - if you can you should believe this answer much more).
'pud' backwards is 'dup'.
'dup' after 'gid' is 'GIDDUP'.
'and' acts as a link.