Border force on bank guarding river (8)
I believe the answer is:
frontier
'border' is the definition.
(I've seen this before)
'force on bank guarding river' is the wordplay.
'force on' becomes 'fon' (I can't explain this - if you can you should believe this answer much more).
'bank' becomes 'tier' (I've seen this before).
'guarding' means one lot of letters goes inside another.
'river' becomes 'r'.
'fon'+'tier'='fontier'
'fontier' going around 'r' is 'FRONTIER'.
Can you help me to learn more?
(Other definitions for frontier that I've seen before include "Land border of a country" , "Wilderness at the edge of a settled area" , "Verge" , "National border" , "Land-border of a country" .)