S is one mistake in it (6)
I believe the answer is:
sierra
's' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'one mistake in it' is the wordplay.
'one' becomes 'i' (Roman numeral).
'mistake' becomes 'err' (to err is to make a mistake or blunder).
'in' is an insertion indicator.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'i'+'err'='ierr'
'ierr' put into 'sa' is 'SIERRA'.
'is' is the link.
Can you help me to learn more?
(Other definitions for sierra that I've seen before include "Mountain chain, S (radio comms.)" , "- - Leone" , "Spanish range of mountains" , "mountain area" , "Mountain range -- code word for S" .)