Is climbing before slip up on a mountain range (6)
I believe the answer is:
sierra
'mountain range' is the definition.
(I know this)
'is climbing before slip up on a' is the wordplay.
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'before' says to put letters next to each other.
'slip up' becomes 'err' (to err is to make an error).
'on' is a charade indicator (letters next to each other).
'is' written backwards gives 'si'.
'si'+'err'+'a'='SIERRA'
(Other definitions for sierra that I've seen before include "Mountain range -- code word for S" , "Andalusian mountain range" , "letter read out on the radio" , "Long range" , "Spanish word for a mountain" .)