Laugh at unknown fodder (3)
I believe the answer is:
hay
'fodder' is the definition.
(I know that hay is a type of fodder)
'laugh at unknown' is the wordplay.
'laugh' becomes 'ha' ().
'at' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'ha'+'y'='HAY'
(Other definitions for hay that I've seen before include "Straw" , "Animal fodder" , "Meadow crop" , "Dried grass as fodder" , "It is to be made while the sun shines" .)