Mother's Day morning (3)
I believe the answer is:
dam
'mother's' is the definition.
(female parent of an animal)
'day morning' is the wordplay.
'day' becomes 'd'.
'morning' becomes 'am'.
'd'+'am'='DAM'
(Other definitions for dam that I've seen before include "Barrier to hold back water" , "Hold back; mother" , "Mother - barrier" , "A mad animal mother" , "Water retainer" .)