I'm assuming you're talking about the WiFiClientSecure.h that's part of the ESP8266WiFi library bundled with the ESP8266 core for Arduino.
That library is written for directly programming the ESP8266. The microcontroller you're attempting to compile for is the ATmega328P. The ESP8266WiFi library is not compatible with the ATmega328P so you can't do this.
zxrycyan:
I'm using an Arduino Uno board with a WiFi shield
Which WiFi shield?