We report on a 63-ks long XMM-Newton observation of the accreting millisecond pulsar SAX J1808.4-3658 during the latest X-ray outburst which started on September 21st 2008 .
The pn spectrum shows a highly significant emission line in the energy band where the iron K- \alpha line is expected , and which we identify as emission from neutral ( or mildly ionized ) iron .
The line profile appears to be quite broad ( more than 1 keV FWHM ) and asymmetric ; the most probable explanation for this profile is Doppler and relativistic broadening from the inner accretion disc .
From a fit with a diskline profile we find an inner radius of the disc of 8.7 ^ { +3.7 } _ { -2.7 } R _ { g } , corresponding to 18.0 ^ { +7.6 } _ { -5.6 } km for a 1.4 M _ { \odot } neutron star .
The disc therefore appears truncated inside the corotation radius ( 31 km for SAX J1808.4-3658 ) in agreement with the fact that the source was still showing pulsations during the XMM-Newton observation .