Most of country upset over one's failure to develop (8)

'develop' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'most of country upset over one's failure' is the wordplay.
'most of' means to remove the last letter.
'country' becomes 'Senegal' (Senegal is an example).
'upset over' says the letters should be written in reverse (knocked over).
'one's failure' becomes 'is' (I can't explain this - if you can you should believe this answer much more).
'senegal' with its final letter taken off is 'senega'.
'senega' backwards is 'agenes'.
'agenes'+'is'='AGENESIS'

'to' acts as a link.

Can you help me to learn more?