19 across
Extra
tests:
restrain
oneself,
turning
tail
(6)
Extra tests: restrain oneself, turning tail (6)
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The definition is at the beginning of the clue.
How the clue works
The definition is Extra tests — exams you have to take again. For the wordplay, take restrain oneself, which gives you RESIST, then "turning tail" means you reverse the last two letters, ST becoming TS, and that lands you on RESITS.