Obviously not the back border (8)
I believe the answer is:
frontier
'border' is the definition.
(I know that border can be written as frontier)
'obviously not the back' is the wordplay.
I cannot really see how this works, but
an anagram of 'not' is 'ont' which is located in the answer.
The remaining letters 'frier' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for frontier that I've seen before include "Border (not yet crossed?)" , "International boundary" , "Verge" , "Border separating two countries" , "Wilderness at the edge of a settled area" .)