The electronic structure modifications by radial deformation are investigated for an armchair (21,21) single-walled carbon nanotube (SWNT), on which scanning tunneling spectroscopy (STS) measurements have recently been made [C. E. Giusca et al., Phys. Rev. B 76, 035429 (2007)]. Plausible radial deformations under hydrostatic pressure are predicted using a semiempirical method based on the continuum elastic shell model [M. Hasegawa and K. Nishidate, Phys. Rev. B 74, 115401 (2006)]. It is found by density-functional electronic-structure calculations that the collapsed (21,21) tube is a semiconductor with small band gap of ≈ 62 meV. This result is consistent with the previous investigations for much thinner armchair SWNTs but contradicts the STS measurements, which indicate that the collapsed tube is metallic with finite density of states at the Fermi level. A possible origin of this contradiction is briefly discussed. [DOI: 10.1380/ejssnt.2009.541]