Pop group ultimately upset country fan (7)
I believe the answer is:
patriot
'country fan' is the definition.
(a patriot is a lover or fan of their country)
'pop group ultimately upset' is the wordplay.
'pop' becomes 'pa' (both can mean father).
'group' becomes 'trio' (a group of three).
'ultimately' indicates one should take the final letters.
The last letter of 'upset' is 't'.
'pa'+'trio'+'t'='PATRIOT'
(Other definitions for patriot that I've seen before include "One supporting country" , "Flag waver" , "One with a strong love of country" , "Nationalist" , "Devotee of their country" .)