GB mining gold, somehow extraordinary (4-8)
I believe the answer is:
mind-boggling
'extraordinary' is the definition.
(I know that extraordinary can be written as mind-boggling)
'gb mining gold somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'gb'+'mining'+'gold'='gbmininggold'
'gbmininggold' anagrammed gives 'MIND-BOGGLING'.
(Other definitions for mind-boggling that I've seen before include "Beyond comprehension" , "Overwhelming" , "Astonishing" , "Incredible" , "Almost impossible to comprehend" .)