Mother cracked up (3)
I believe the answer is:
dam
'mother' is the definition.
(female parent of an animal)
'cracked up' is the wordplay.
'cracked' becomes 'mad' (cracked can informally mean mad).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'mad' backwards is 'DAM'.
(Other definitions for dam that I've seen before include "Barrier to hold back water" , "Wall restraining water" , "River restraint" , "Mother of quadruped" , "Horse's mother" .)