Pole cracks up (4)
I believe the answer is:
spar
'pole' is the definition.
(spar is a kind of pole)
'cracks up' is the wordplay.
'cracks' becomes 'raps' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'raps' back-to-front is 'SPAR'.
(Other definitions for spar that I've seen before include "Box without punching hard" , "Scrap" , "Round pole to support sail" , "Box (lightly) - fight (verbally)" , "Box in training - mineral" .)