How to make this code fire projectile once during the Update function? I know Update means every frame.

I use the code below for the enemy to fire at the player. I can make it fire only once using a key but how can it fire only once without a key? Where should a boolean be placed?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class FireAtPlayer : MonoBehaviour {
    bool p_detected;
    public Transform target;
    public UnityEngine.AI.NavMeshAgent agent;
    public Rigidbody projectile;
    public Transform spawn;
    public int speed;

    // Use this for initialization
    void Start () {
        speed = 60;
	// Update is called once per frame
	void Update () {
        p_detected = GetComponentInChildren<PlayerDetection>().player_detected;
        if (p_detected)
            target = GameObject.FindGameObjectWithTag("Player").transform;
            GetComponent<walking>().enabled = false;
            agent.isStopped = true;
        if (!p_detected)
            target = null;
            GetComponent<walking>().enabled = true;
            agent.isStopped = false;

    IEnumerator shooting()
        yield return new WaitForSeconds(3);
        var clone = Instantiate(projectile, spawn.position, transform.rotation);
        var direction = (target.transform.position - clone.transform.position).normalized;
        clone.velocity = direction * speed;

Just count the time and check if you passed a whole second. This can be done by either accumulating Time.deltaTime and see when it passed 1, or by saving Time.time once and subtracting that from Time.time each frame to see when that difference is 1.
If the corresponding condition is met, reset.