Clue: Practising linguist of old making wayward sister repent (12)
Answer: INTERPRETESS
Explanation: Definition: Practising linguist of old
(SISTER REPENT)* – strangely Chambers marks this spelling as "obs", but thinks (the more regularly formed – cf waiter/waitress) "interpretress" is fine: surely no one has use such an expression nowadays (the acceptability of female-marked -ess forms is an interesting topic: the waiter/waitress distinction is still going strong, but "actress" is losing ground, and words such as "authoress" and "poetess" seem distinctly old-fashioned, not to mention a bit patronising)