We present a precise timing analysis of the accreting millisecond pulsar XTE J1814–338 during its 2003 outburst , observed by RXTE . A full orbital solution is given for the first time ; Doppler effects induced by the motion of the source in the binary system were corrected , leading to a refined estimate of the orbital period , P _ { orb } = 15388.7229 ( 2 ) s , and of the projected semimajor axis , a \sin i / c = 390.633 ( 9 ) lt-ms. We could then investigate the spin behaviour of the accreting compact object during the outburst . We report here a refined value of the spin frequency ( \nu = 314.35610879 ( 1 ) Hz ) and the first estimate of the spin frequency derivative of this source while accreting ( \dot { \nu } = ( -6.7 \pm 0.7 ) \times 10 ^ { -14 } Hz/s ) . This spin down behaviour arises when both the fundamental frequency and the second harmonic are taken into consideration . We discuss this in the context of the interaction between the disc and the quickly rotating magnetosphere , at accretion rates sufficiently low to allow a threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity . We also present indications of a jitter of the pulse phases around the mean trend , which we argue results from movements of the accreting hotspots in response to variations of the accretion rate .