Mom's mom (4)
I believe the answer is:
gran
'mom's mom' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gran that I've seen before include "Senior family member" , "... Canaria in the Atlantic Ocean" , "old lady" , "... Canaria, island off African coast" , "Mum's mum" .)