Knife in her egg a diner's taken back (6)
I believe the answer is:
dagger
'knife' is the definition.
(I know that dagger is a type of knife)
'in her egg a diner's taken back' is the wordplay.
'in' indicates a hidden word.
'taken back' shows that the letters should be reversed in order.
'reggad' can be found hidden inside 'her egg a diners'.
'reggad' back-to-front is 'DAGGER'.
(Other definitions for dagger that I've seen before include "'Short, double-edged blade (6)'" , "A weapon; a reference mark" , "Macbeth's weapon" , "The weapon Macbeth thought he saw" , "Stabbing weapon" .)