Ace is going the wrong way, mistake being admitted in mountain range (6)
I believe the answer is:
sierra
'mountain range' is the definition.
(sierra is a kind of mountain range)
'ace is going the wrong way mistake being admitted' is the wordplay.
'ace' becomes 'a'.
'going the wrong way' says the letters should be written backwards.
'mistake' becomes 'err' ('err' can be a synonym of 'mistake'**).
'being admitted' is an insertion indicator.
'a'+'is'='ais'
'ais' back-to-front is 'sia'.
'sia' enclosing 'err' is 'SIERRA'.
'in' acts as a link.
(Other definitions for sierra that I've seen before include "Comms code S" , "------ Nevada, mountains of Spain and California" , "- Nevada, Spanish mountain range" , "A Spanish mountain chain (6)" , "Spanish word for a mountain" .)