Definition of "nina"

The puzzle itself was fairly easy however there was an elegant nina based on the middle entry (INSIDE STORY) in which each of the across answers hid a synonym of "story", e.g. 5A and 6A were: STABle gendER.

2008 April 1, Ilan Caron, “Guardian Cryptic Crossword No. 24350 set by Brendan”, in rec.puzzles.crosswords‎ (Usenet)

Ninas may or may not be present and, even if one is there, it can go unnoticed because, not being intrinsic to the solution, its presence is not advertised—or because it is so well hidden.

2014 August 10, Brian Bethune, “The crossword’s meandering 100-year journey”, in Maclean's‎

Other “personal” ninas are happier and subtler: birthday wishes to loved ones, which, ultimately, have only one intended reader

2014, Alan Connor, The Crossword Century: 100 Years of Witty Wordplay, Ingenious Puzzles, and Linguistic Mischief, Penguin