Fundamental way to get in (3)
I believe the answer is:
key
'fundamental' is the definition.
(I know that fundamental can be written as key)
'way to get in' is the wordplay.
'way' becomes 'e' (Compass direction - East).
'to get' becomes 'ky' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'e' inserted into 'ky' is 'KEY'.
Can you help me to learn more?
(Other definitions for key that I've seen before include "One that locks" , "locker" , "Essential; solution" , "Of crucial importance" , "It can open" .)