'a definite concern' is correct and can be used in written English.
It is usually used in the context of expressing a issue or concern for something that has been observed or experienced. For example: "The recently observed rise in temperature is a definite concern that must be addressed with urgency."
"Money is a definite concern every time I break a twenty," Mrs. Morrison said.
"There is a definite concern about 'lights out London,' " she said, "but the reality is that half of what is bought is bought for rental".
"There is a definite concern that we're going to lose capacity with older plants shutting down just as we're adding this new capacity, and the demand has kept growing," he said.
I've also had kidney issues in the past so that is a definite concern.
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"Because there's such a close proximity of the residences to the school site, there's a definite concern that the stadium is going to have a very negative impact on the community," she said.
And yet, though many of us would place ourselves in the former camp, there remains a definite niggling concern that yet again English football is failing to regard football management with the gravitas it merits.
However, in an older man without prostate cancer, there is at least a small amount of evidence that testosterone will increase bone density, but there is definite concern about potential side effects.
While there is definite concern about concussions, as polling data shows, fans just haven't been ready to pull the plug.
Severe (Grade 3): the adverse event interfered considerably with the patient's usual activities and was of definite concern to the patient and/or posed substantial risk to the patient's health or well-being.
It was anticipated that the small number in this subgroup would not allow a definite conclusion concerning non-inferiority and no power calculation was performed.
