We report the discovery and analysis of PTF1 J085713+331843, a new eclipsing post-common-envelope detached white-dwarf-red-dwarf binary with a 2.5 h orbital period discovered by the Palomar Transient Factory. ULTRACAM multicolour photometry over multiple orbital periods reveals a light curve with a deep flat-bottomed primary eclipse and a strong reflection effect. Phase-resolved spectroscopy shows broad Balmer absorption lines from the DA white dwarf and phase-dependent Balmer emission lines originating on the irradiated side of the red dwarf. The temperature of the DA white dwarf is TWD = 25 700 ± 400 K and the spectral type of the red dwarf is M3-M5. A combined modelling of the light curve and the radial velocity variations results in a white dwarf mass of M_WD = 0.61^{+0.18}_{-0.17} M_{⊙} and radius of R_WD = 0.0175^{+0.0012}_{-0.0011} R_{⊙}, and a red dwarf mass and radius of M_RD = 0.19^{+0.10}_{-0.08} M_{⊙} and R_RD = 0.24^{+0.04}_{-0.04} R_{⊙}. The system is either a detached cataclysmic variable or has emerged like from the common-envelope phase at nearly its current orbital period. In {˜ }70 Myr, this system will become a cataclysmic variable in the period gap.