'only a proportion' is a correct and usable phrase in written English.
It can be used to describe a portion of a larger group or amount. For example: "Only a proportion of the population have access to affordable healthcare."
Police seized 56 Baikal pistols supplied by the gang, which was "only a proportion" of the true figure.
For people who need residential care this means that only a proportion of those costs are actually accounted for by care.
Only a proportion of patients at risk will respond to therapy.
Of note, only a proportion of mice (37 %) underwent ventricular dilatation with improved survival.
However, only a proportion of patients actually respond to ECP and parameters predictive of response need to be discovered.
A probable explanation of this modest reduction is that only a proportion of patients with unipolar depression are diagnosed, and only a proportion of those patients are treated, mostly with antidepressants.
This suggests that there may be an optimum strategy for secreting chemicals that could explain why many unpalatable insects secrete only a proportion of their defence chemicals.
Clinical disease affects only a proportion of infected dogs and a complex genetic background of immune response is responsible for this susceptibility.
Sixth, selection bias was introduced, as our study group consisted of only a proportion of all patients undergoing MR of the knee at our institution.
The mass flux of molecules impacting the liquid surface is J i ; however, only a proportion of these molecules, J c, actually condense into the liquid.
Conclusions: Intensivists spend considerable clinical time reviewing patients on general wards, however only a proportion (26.6 %) require admission to an intensive care unit.
