In the last decade, a new class of pulsars, called transitional millisecond pulsars (tMSPs), has emerged. These systems switch between a low-mass X-ray binary and a radio millisecond pulsar state, establishing a link between the two source classes but also raising new puzzles. tMSPs provide a great avenue to study the low-level accretion processes that spin-up pulsars to millisecond periods. We have timed the tMSP J1023+0038, in both the radio state (prior to 2013) and X-ray state (since 2013) over multi-year baselines. In the radio state, orbital variations and eclipses complicate the timing model. In the X-ray pulsar state, we have timed J1023 in a dedicated XMM-Newton campaign. In this campaign, we have, for the first time, been able to precisely measure the spin period derivative of a tMSP in the accreting state. We find that only a small additional torque is being exerted on the pulsar. In my talk I will present our technique and, discuss the implications of our result for understanding the nature of the accretion flow and its influence on the pulsar’s magnetosphere.