Polyhedron on board problem sir put up (5)
I believe the answer is:
prism
'polyhedron' is the definition.
(prism is a kind of polyhedron)
'on board problem sir put up' is the wordplay.
'on board' indicates a hidden word (the answer is contained by or 'on board' the clue).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'msirp' is hidden within 'problem sir put'.
'msirp' reversed gives 'PRISM'.
(Other definitions for prism that I've seen before include "What's on Dark Side of the Moon" , "Glass triangle that separates white light into the colour spectrum" , "something that colours views?" , "Light transmitter" , "Transparent solid for dispersing light" .)