Magnificent ravine not half famous (8)
I believe the answer is:
gorgeous
'magnificent' is the definition.
(I've seen this before)
'ravine not half famous' is the wordplay.
'ravine' becomes 'gorge' (gorge is a kind of ravine).
'not half' suggests halving the letters.
'famous' halved is 'ous'.
'gorge'+'ous'='GORGEOUS'
(Other definitions for gorgeous that I've seen before include "Extremely beautiful" , "Lovely" , "Showy" , "Strikingly beautiful" , "Dazzlingly beautiful" .)