Really losing regular characters in American mountain range (5)
I believe the answer is:
urals
'mountain range' is the definition.
(Russian mountain range)
'really losing regular characters in american' is the wordplay.
'losing regular characters' indicates alternate letters (letters removed at regular intervals).
'in' indicates putting letters inside.
'american' becomes 'us' (abbreviation for United States).
The alternate letters of 'really' are 'ral'.
'ral' inserted inside 'us' is 'URALS'.
(Other definitions for urals that I've seen before include "Mountains on the boundary between Europe and Asia" , "mountain area" , "Mountains of west-central Russia" , "Russian mountains" , "Mountain range in Russia and Kazakhstan" .)