Devon Still's Ex -- Child Support War Is Over ... 'We're Working Together'

Devon Still's ex is waving the white flag in their fight over child support -- telling TMZ Sports the two sides have reached a financial agreement that's in the best interest for their daughter Leah.

The Cincinnati Bengals player -- whose 4-year-old daughter is ferociously battling cancer -- had been at odds with Leah's mother ... who claimed Devon wasn't living up to his end of a "voluntary" child support arrangement they struck over the summer.

Leah's mother, Channing Smythe, claimed she needed the cash for living expenses ... saying she couldn't work because caring for Leah is a full-time job in itself.

But now, Channing says the two sides were able to work out their money issues -- telling us, "He is helping me out financially" ... noting that Devon is still playing a huge role in parenting Leah.

"I am very happy that we are working together for Leah."