The putter is the problem (5)
'problem' is the definition.
(poser is a kind of problem)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for poser that I've seen before include "Puzzle using ropes", "Someone behaving so as to be noticed", "Difficult problem", "Tricky clue", "Baffling question or problem".)