Uninterrupted ascent is unusual round face of Nevis (9)
I believe the answer is:
incessant
'uninterrupted' is the definition.
(thesaurus)
'ascent is unusual round face of nevis' is the wordplay.
'unusual' is an anagram indicator.
'round' is an insertion indicator.
'face of' suggests taking the first letters.
The first letter of 'nevis' is 'n'.
'ascent'+'is'='ascentis'
'ascentis' is an anagram of 'incessat'.
'incessat' placed around 'n' is 'INCESSANT'.
(Other definitions for incessant that I've seen before include "Nine casts (anag) - nonstop" , "Without pausing" , "Instances (anag)" , "Endless" , "Continual and uninterrupted" .)