Ed Cowan will captain the side in Clarke's absence, while another Ashton from Western Australia, 20-year-old Ashton Turner, has been called into the team to make his first-class debut.

Clarke pulled up stiff and sore from the second Test and will be given an opportunity to rest up ahead of the third Test at Old Trafford starting next Thursday.

Turner and 12th man for the Sussex match, Travis Head, are with the Australian team on scholarship with the AIS Centre of Excellence.

The pair are development players on tour and have been based in Hampshire, but have joined the Australian camp at various stages through the tour.

Turner is a right-arm offspinner who played under 19s cricket for Australia alongside Ashton Agar, who will also play.

Agar picked up a slight hip strain during the second Test at Lord's, but has pulled up fine and will be right to bowl in the three-day match.

Turner and Agar's selection means Australia are playing three spinners against Sussex, with Nathan Lyon also getting a chance to press his claims for the third Test.

Turner was part of Australia's under 19s team that were runners up to India at last year's World Cup.

Australia only have 10 Ashes squad players available for the match, with James Pattinson ruled out for the remainder of the tour, David Warner playing for Australia A in South Africa, and several other players rested.

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Cowan will open the batting with Phil Hughes, while wicketkeeper Matthew Wade will play his first match for the tour.

Haddin is the first choice keeper, but given Australia's batting woes, Wade has the chance to push for selection as a specialist batsman if he scores runs.

Mitchell Starc, Jackson Bird and James Faulkner form the fast bowling attack.

The trio is effectively bowling-off for the final position in the attack to complement Siddle and Harris at Old Trafford.

Australian team to play Sussex: Ed Cowan (capt), Phil Hughes, Usman Khawaja, Steve Smith, Matthew Wade, James Faulkner, Ashton Agar, Ashton Turner, Mitchell Starc, Nathan Lyon, Jackson Bird.