European country investing in oil mostly upset Nigerian people (4)

'european country investing in oil mostly upset nigerian people' is the definition.
I can't judge whether this defines the answer.

'european country investing in oil mostly upset' is the wordplay.
'european country' becomes 'gb' (Great Britain. I am not sure about the 'european' bit.).
'investing in' indicates putting letters inside.
'mostly' means to remove the last letter.
'upset' shows that the letters should be reversed in order.
'oil' with its last letter taken away is 'oi'.
'oi' back-to-front is 'io'.
'gb' going inside 'io' is 'IGBO'.

Can you help me to learn more?