It keeps one blunder within range (6)
I believe the answer is:
sierra
'range' is the definition.
(sierra is a kind of range)
'it keeps one blunder within' is the wordplay.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'keeps' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'blunder' becomes 'err' (I've seen this before).
'within' is an insertion indicator.
'sa' placed around 'i' is 'sia'.
'sia' enclosing 'err' is 'SIERRA'.
(Other definitions for sierra that I've seen before include "Long range" , "A Spanish mountain chain (6)" , "Jagged mountain range" , "Jagged mountains - code word for S" , "Spanish word for a mountain" .)