Laugh at oddly rude firm (4)
I believe the answer is:
hard
'firm' is the definition.
(both can mean durable or strong)
'laugh at oddly rude' is the wordplay.
'laugh' becomes 'ha'.
'at' is a charade indicator (letters next to each other).
'oddly' means one should take alternating letters.
The alternate letters of 'rude' are 'rd'.
'ha'+'rd'='HARD'
(Other definitions for hard that I've seen before include "Tough - laborious" , "Novel featuring Mr Gradgrind" , "badly" , "Solid; difficult" , "Difficult; rigid" .)