No sure if you are on the mobile or web version of the game so I have included both below.
Click on the EQUIPMENT tab from your Profile page on the website. From there you will see a button titled SELL EQUIPMENT. Click that button to begin the process of selecting which items to sell.
Please keep in mind that Equipment that you can sell to the Pro Shop will be listed on the page and will have a checkbox for you to click if you want to sell the item. If a club is currently in your bag the word "Equipped" will be displayed. You cannot sell back a club that is in your bag. You will need to remove the club from your bag before you will be allowed to sell the item. The total value you will receive for the equipment you have selected will be listed in the TOTAL VALUE box at the bottom of the page.
To sell your clubs, select your profile photo in the top right of the main menu then select EDIT EQUIPMENT.
At the bottom of the Equipment box is SELL CLUBS. Choose this option to sell items back to the proshop.