Under external electric field, a free standing silicene monolayer is investigated for the physical and electronic features using the density functional theory with norm-conserving pseudopotentials and pseudo-atomic localized basis functions. It is found that silicene tends to be flat instead of buckled as applying electric field perpendicular to the surface. We observed on some range magnitude of electric field and noticed that there is a monotonic decreasing in number of DOS near the Fermi level showing the possibility of tunable bandgap on silicene. We also found that in this higher magnitude electric field, the Dirac cone is no longer occurring at K-point while the band gap term remains direct. The asymmetry onsite potential between the Si atom at site A and B spoils the degeneracy at the K point by the presence of external or internal influence. In this work, we try to combine those influences by considering Stone Wales (SW) defect and outer z-direction of electric field as internal and external factors respectively. A non-linear correlation of the result is profoundly becoming more effective way in effort to bring remarkable band-gap in silicene monolayer.