Car — I slip into it (6)
I believe the answer is:
sierra
'car' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'i slip into it' is the wordplay.
'slip' becomes 'err' (to err is to make an error).
'into' is an insertion indicator.
'it' becomes 'sa' (SA can mean sex appeal, as in 'it').
'i'+'err'='ierr'
'ierr' going within 'sa' is 'SIERRA'.
Can you help me to learn more?
(Other definitions for sierra that I've seen before include "Spanish word for a mountain" , "Long jagged mountain chain" , "Comms code S" , "------ Nevada, mountains of Spain and California" , "Jagged mountain range" .)