Cheese substitute's last used by mother (4)
I believe the answer is:
edam
'cheese' is the definition.
(I've seen this before)
'substitute's last used by mother' is the wordplay.
'last' says to take the final letters.
'used by' says to put letters next to each other.
'mother' becomes 'dam' (female parent of an animal).
The final letter of 'substitute' is 'e'.
'e'+'dam'='EDAM'
(Other definitions for edam that I've seen before include "something to have with crackers" , "Netherlands cheese" , "edible sphere" , "Hard Dutch cheese" , "Round, yellow cheese" .)