Welcome to FT

The problem is that when you call to explain that you can't make it to the flight, the agent will most likely also insist to rebook and reprice the ticket.
Its is worth a try but be prepared that it may not work and you have to pay for rebooking the ticket.

Any chance that you can wait for a schedule change or cancellation of the flight in question ? (which will allow you to ask the airline to remove that leg from the itinerary).

If you just no-show for the flight most likely your return flight will be cancelled.