Plants often modify microsite conditions important for seedling establishment. In tropical moist forest, advance regeneration in the form of shade-suppressed seedlings is a major component of regrowth in new gaps. Tree seedlings may be filtered by the composition and structure of the forest understorey. In a lowland forest in Central Panama, we examined light availability, litter accumulation and the seedling community (abundance, diversity and composition) under and away from the canopies of a common subcanopy, clonal palm, Oenocarpus mapora, and tested whether seedling abundance varies as a function of changes in understorey light conditions and leaf litter depth. We found evidence that leaf litter was five times deeper and light availability 27% lower under O. mapora canopy than where it was absent. The probability of finding a seedling in plots under O. mapora canopies was 33% lower than in plots without the palm. Plots under O. mapora canopies also had 49% fewer species of seedlings and those seedlings came from significantly larger seeds than seedlings in plots not under the palm. Overall, seedling density was positively correlated with light availability and negatively correlated with leaf litter depth. A transplant experiment confirmed that for at least one large-seeded species of dicot, seedlings are negatively affected by the reduction in light availability and increase in leaf litter. Under O. mapora, seedlings of Gustavia superba (Lecythidaceae) had lower survivorship and biomass accumulation than away from the palm. Thus, site conditions produced by large, clonal, multi-layered palms such as O. mapora may reduce establishment of small-seeded and shade-intolerant species, potentially affecting species composition in the understorey seedling pool.