"a narrow portion" is correct and usable in written English.
For example, you might write "The river narrowed to a narrow portion at the bend."
But I wonder if the approach will only appeal to a narrow portion of the community".
The ruling focused on a narrow portion of the legal challenge dealing with modified-risk tobacco products.
Most of groundwater flow and pollutant flux discharges through a narrow portion near the top of seepage face.
Coastal marshes in Louisiana are inherently vulnerable to these threats because they are microtidal and inhabit a narrow portion of the intertidal zone.
Fluxes were concentrated within a narrow portion of the slope surface near the intersection point between the lake water level and the slope face.
Each column can only detect light from a narrow portion of an arthropod's field of vision.
We looked at a narrower portion of gun violence than The New York Times did, only counting incidents in which four people were fatally shot, not including the shooter.
The COP was generally situated posterior to the metatarsal heads in a narrower portion of the foot.
Although RTVue AS-OCT produces higher resolution images than Visante AS-OCT, it can only capture a narrower portion of the anterior chamber.
There is also evidence of the exopods in Cheloniellon being differentiated into proximal and distal lobes; the flap-like lobe clearly inserts into a narrower portion in figures 8a, 11 of Stürmer and Bergström [ 62]. 15.
That state park is now an island, because Michael's storm surge blasted a breach in an especially narrow portion of the peninsula.
