Cross mother up (3)
I believe the answer is:
mad
'cross' is the definition.
(thesaurus)
'mother up' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'dam' backwards is 'MAD'.
(Other definitions for mad that I've seen before include "Extremely foolish" , "Frenzied" , "Crazy" , "Roused to anger" , "Deranged" .)