One mountain restricting ways forward (8)
I believe the answer is:
immodest
'forward' is the definition.
(thesaurus)
'one mountain restricting ways' is the wordplay.
'one' becomes 'i' (Roman numeral).
'mountain' becomes 'mt'.
'restricting' indicates putting letters inside.
'ways' becomes 'modes' ('mode' can be a synonym of 'way'**).
'mt' going around 'modes' is 'mmodest'.
'i'+'mmodest'='IMMODEST'
(Other definitions for immodest that I've seen before include "Offending against sexual mores in appearance" , "Brazen" , "Indecent or shameless, perhaps in dress" , "Lacking decency, shameless" , "blowing its own trumpet" .)