Curtail break and work to go over puzzle (5)
I believe the answer is:
Here is my best explanation:
'puzzle' is the definition.
(I've seen this before)
'curtail break and work to go over' is the wordplay.
'curtail' means to remove the last letter (I've seen 'curtailed' mean this).
'break' becomes 'rest' (rest is a kind of break).
'and' says to put letters next to each other.
'work' becomes 'op' (abbreviation for opus).
'to go over' says the letters should be written backwards.
'rest' with its final letter taken away is 'res'.
'resop' reversed gives 'poser'.
(Other definitions for poser that I've seen before include "Puzzling question for the model", "difficult puzzle", "A model", "Model - problem", "Problem set by the model", "Hard question", "Tricky problem", "The attitudiniser", "Spore raises a difficult question" and "Puzzling problem".)