Fundamental shift? (3)
I believe the answer is:
key
This clue is a double definition.
'fundamental' is the first definition.
(I know that fundamental can be written as key)
'shift?' is the second definition.
(shift key from a computer keyboard)
(Other definitions for key that I've seen before include "explanation" , "It opens the door" , "Eg, Ctrl, Alt or Esc" , "Coral reef off southern coast of Florida" , "Crucial; island" .)