Senior relative not completely bananas (4)
I believe the answer is:
nana
'senior relative' is the definition.
(nana can mean a grandmother)
'not completely bananas' is the wordplay.
'not completely' indicates the answer is hidden within the clue.
'NANA' is hidden within 'bananas'.
(Other definitions for nana that I've seen before include "Relative" , "Grandma" , "Old woman" , "Zola's heroine" , "granny" .)