  CONCERTO is the first experiment to perform a [CII] line intensity mapping
survey to target $z>5.2$. Measuring the [CII] power spectrum allows us to study
the role of dusty star-forming galaxies in the star formation history during
the Reionization and post-Reionization. The main obstacle to this measurement
is the contamination by bright foregrounds. We evaluate our ability to retrieve
the [CII] signal in mock observations using the Simulated Infrared Dusty
Extragalactic Sky. We compared two methods for dealing with the dust continuum
emission from galaxies: the standard PCA and the arPLS method. For line
interlopers, the strategy relies on masking low-redshift galaxies using
external catalogues. As we do not have observations of CO or classical CO
proxies ,we relied on the COSMOS stellar mass catalogue. To measure the power
spectrum of masked data, we adapted the P of K EstimatoR and discuss its use on
LIM data. The arPLS method achieves a reduction of the continuum background to
a sub-dominant level of the [CII] at z=7 by a factor of>70. When using PCA,
this factor is only 0.7. The masking lowers the power amplitude of line
contamination down to $2 \times 10^2 Jy^2/sr$ This residual level is dominated
by faint undetected sources. For our [CII] model, this results in a detection
at z = 5.2 with a power ratio [CII]/(residual interlopers) = $62 \pm 32$ for a
22 % area survey loss. However, at z = 7, [C II ] / (residual interlopers)$=2.0
\pm 1.4$. Thanks to the large area covered by SIDES-Uchuu, we show that the
power amplitude of line residuals varies by 12-15% for z=5.2-7. We present an
end-to-end simulation of the extragalactic foreground removal that we ran to
detect the [CII] at high redshift via its power spectrum. We show that dust
continuum emission are not a limiting foreground for [CII] LIM. Residual CO and
[CI] limits our ability to measure the [CII] power spectrum at z>7.
