A sibling beyond obnoxious originally in band (5)
I believe the answer is:
oasis
'band' is the definition.
(I have seen 'British band' mean 'oasis' so perhaps 'band' could also mean 'oasis')
'a sibling beyond obnoxious originally' is the wordplay.
'sibling' becomes 'sis' (short for sister).
'beyond' means one lot of letters go next to another.
'originally' says to take the initial letters.
The initial letter of 'obnoxious' is 'o'.
'a'+'sis'='asis'
'asis' put after 'o' is 'OASIS'.
'in' acts as a link.
(Other definitions for oasis that I've seen before include "Peaceful area away from trouble" , "Safe haven" , "Fertile place in a desert, a watering hole" , "Noel & Liam's band" , "Place of rest in the midst of gloom" .)