Small mountain is the most difficult to conquer (8)
I believe the answer is:
severest
'the most difficult to conquer' is the definition.
'small mountain' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mountain' becomes 'everest' (I've seen this before).
's'+'everest'='SEVEREST'
'is' acts as a link.
(Other definitions for severest that I've seen before include "Most strict and harsh" , "Most dangerous" , "Most extreme or difficult" , "Most harsh and punitive" .)