The formation of the first virialized structures in overdensities dates back to ~9 Gyr ago, i.e. in the redshift range z ~ 1.4 - 1.6. Some models of structure formation predict that the star formation activity in clusters was high at that epoch, implying large reservoirs of cold molecular gas. Aiming at finding a trace of this expected high molecular gas content in primeval clusters, we searched for the 12CO(2-1) line emission in the most luminous active galactic nucleus (AGN) of the cluster around the radio galaxy 7C 1756+6520 at z ~ 1.4, one of the farthest spectroscopic confirmed clusters. This AGN, called AGN.1317, is located in the neighbourhood of the central radio galaxy at a projected distance of ~780 kpc. The IRAM Plateau de Bure Interferometer was used to investigate the molecular gas quantity in AGN.1317, observing the 12CO(2-1) emission line. We detect CO emission in an AGN belonging to a galaxy cluster at z ~ 1.4. We measured a molecular gas mass of 1.1 x 10^10 Msun, comparable to that found in submillimeter galaxies. In optical images, AGN.1317 does not seem to be part of a galaxy interaction or merger.We also derived the nearly instantaneous star formation rate (SFR) from Halpha flux obtaining a SFR ~65 Msun/yr. This suggests that AGN.1317 is actively forming stars and will exhaust its reservoir of cold gas in ~0.2-1.0 Gyr. ; Comment: 5 pages, 3 figures, accepted for publication in Astronomy & Astrophysics