We present a stepwise in situ filling of metallic single-walled carbon nanotubes (SWCNTs) with a mean diameter of 1.4 nm with metallocenes and a concomitant characterisation of their electronic properties using photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) without breaking the ultra-high vacuum environment. The changes in the SWCNTs' electronic structure upon ferrocene (FeCp2) filling are suggested by a broadening of the C 1s spectral line. Weak n-type charge transfer from the ferrocene to the SWCNT is evidenced by a reduction of the intensity of the first van Hove singularity (vHs) in the fine structure of the unoccupied density of states as well as an increase in the valency of Fe in the FeCp2 molecule. Additionally, we evidence the presence of clean ferrocene with a resonance photoemission study showing the appearance of electronic states of localised Fe 3d electrons. This gives further insight into the electrical properties of functionalised carbon nanotubes fabricated in a clean environment avoiding inclusion of impurities.