SPI Slave Programming for Arduino
SPI.h library by default only support Master mode, the SPI.begin() will automatically set slave select (ss) pin to HIGH whether you have pinMode(ss, OUTPUT) in your setup() code or not and set the Arduino as an SPI master. This can be see on SPI library source code.
As the SPI library does not contain the functionality for slave mode, in order to use Arduino as a Slave, you will need to do some customised setup, mainly on 1) setting MISO as OUTPUT for slave to send data; 2) enable slave mode by setting the SPCR register; 3) enable interrupt when data is received. Here is an example for an SPI slave on Arduino.
#include&lt;SPI.h&gt;

volatile boolean received;
volatile byte receivedData;

ISR (SPI_STC_vect)        //Inerrrput routine function 
{
  receivedData = SPDR;   // Get the received data from SPDR register
  received = true;       // Sets received as True 
}

void setup()
{
  Serial.begin(115200);

  pinMode(MISO,OUTPUT);   //Sets MISO as OUTPUT
  SPCR |= _BV(SPE);       //Turn on SPI in Slave Mode
  received = false;
  SPI.attachInterrupt();  //Activate SPI Interuupt 
}


void loop()
{ 
  if(received) {                        
    SPDR = receivedData;    // send back the received data, this is not necessary, only for demo purpose
    received = false;
    Serial.print(receivedData, HEX);
  }
}