FLASH early science - discovery of an intervening HI 21-cm absorber from an ASKAP survey of the GAMA 23 field by Allison, JR, Sadler, EM, Bellstedt, S, Davies, LJM, Driver, SP, Ellison, SL, Huynh, M, Kapinska, AD, Mahony, EK, Moss, VA, Robotham, ASG, Whiting, MT, Curran, SJ, Darling, J, Hotan, AW, Hunstead, RW, Koribalski, BS, Lagos, CDP, Pettini, M, Pimbblet, KA, Voronkov, MA