State upset Queen's brief (5)
I believe the answer is:
niger
'state upset queen's brief' is the definition.
'niger' can be an answer for 'state' (I have seen 'African state ' mean 'niger' so perhaps 'state' could also mean 'niger'). I am unsure of the 'upset queen's brief' bit.
'state upset queen's brief' is the wordplay.
'state' becomes 'gin' (I can't explain this - if you can you should believe this answer much more).
'upset' says the letters should be written in reverse.
'queen's' becomes 'ers' (abbreviation for Elizabeth Regina).
'brief' means to remove the last letter (a shorter or briefer version of the word).
'gin' in reverse letter order is 'nig'.
'ers' with its last letter taken off is 'er'.
'nig'+'er'='NIGER'
Can you help me to learn more?
(Other definitions for niger that I've seen before include "22 across (anag) -- northwest African river" , "African state and river" , "West African republic" , "African country dominated by the Sahara, until 1969 a French colony" , "W. African country" .)