No downs could possibly be mountainous! (7)
I believe the answer is:
snowdon
'mountainous' is the definition.
'no downs could possibly' is the wordplay.
'could possibly' is an anagram indicator.
'no'+'downs'='nodowns'
'nodowns' anagrammed gives 'SNOWDON'.
'be' acts as a link.
(Other definitions for snowdon that I've seen before include "Welsh mountain" , "Highest mountain in Wales" , "High spot in Wales" , "Highest Welsh peak" , "Highest peak in Wales" .)