Flute-type fluctuations excited spontaneously at the edge of a strongly magnetized plasma column couple nonlinearly with an externally injected convective cell. During the interaction the azimuthal mode-number spectrum of the fluctuations is strongly deformed, and a broader spectrum appears. The phase and amplitude of the fluctuations vary after the interaction and eventually an ordered structure is formed with a phase locked to the injected cell. The macroscopic behaviour is analyzed numerically, using a simple model equation of the Van der Pol type. The observed ordering of the fluctuations is well explained by this type of equation, both for the case when the fluctuations consist of a single mode and when they consist of two modes of different frequencies.