Here, we present Hubble Space Telescope/Advanced Camera for Surveys (ACS) imaging, in the B and I bands, of the edge-on Sb/Sc galaxy NGC 5170. Excluding the central disc region, we detect 142 objects with colours and sizes typical of globular clusters (GCs). Our main result is the discovery of a 'blue tilt' (a mass-metallicity relation), at the 3σ level, in the metal-poor GC subpopulation of this Milky Way like galaxy. The tilt is consistent with that seen in massive elliptical galaxies and with the self-enrichment model of Bailin & Harris. For a linear mass-metallicity relation, the tilt has the form Z ~L 0.42±0.13. We derive a total GC system population of 600 ± 100, making it much richer than the Milky Way. However, when this number is normalized by the host galaxy luminosity or stellar mass it is similar to that of M31. Finally, we report the presence of a potential Ultra Compact Dwarf of size ~6 pc and luminosity M1 ~ -12.5, assuming it is physically associated with NGC 5170.