Whisper sister will eat this last (7)
I believe the answer is:
persist
'this last' is the definition.
(I've seen this in another clue)
'whisper sister will eat' is the wordplay.
'eat' says the answer is hidden in the clue (I've seen 'eating' mean this).
'PERSIST' is hidden within 'whisper sister will'.
(Other definitions for persist that I've seen before include "Continue (in spite of difficulties)" , "Pursue course doggedly" , "Stick with" , "Continue in spite of resistance" , "Prevail" .)