We report on the young massive clump ( G35.20w ) in W48 that previous molecular line and dust observations have revealed to be in the very early stages of star formation .
Based on virial analysis , we find that a strong field of 1640 \leavevmode \nobreak { } \mu { } G is required to keep the clump in pressure equilibrium .
We performed a deep Zeeman effect measurement of the 113 GHz CN ( 1-0 ) line towards this clump with the IRAM 30 m telescope .
We combine simultaneous fitting of all CN hyperfines with Monte Carlo simulations for a large range in realization of the magnetic field to obtain a constraint on the line-of-sight field strength of -687 \pm 420 \leavevmode \nobreak { } \mu { } G .
We also analyze archival dust polarization observations towards G35.20w .
A strong magnetic field is implied by the remarkably ordered field orientation that is perpendicular to the longest axis of the clump .
Based on this , we also estimate the plane-of-sky component of the magnetic field to be \sim 740 \leavevmode \nobreak { } \mu { } G .
This allows for a unique comparison of the two orthogonal measurements of magnetic field strength of the same region and at similar spatial scales .
The expected total field strength shows no significant conflict between the observed field and that required for pressure equilibrium .
By producing a probability distribution for a large range in field geometries , we show that plane-of-sky projections are much closer to the true field strengths than line-of-sight projections .
This can present a significant challenge for Zeeman measurements of magnetized structures , even with ALMA .
We also show that CN molecule does not suffer from depletion on the observed scales in the predominantly cold and highly deuterated core in an early stage of high-mass star formation and is thus a good tracer of the dense gas .