How does new expert practice in international organisations (IOs) emerge and gain recognition? This study analyses the evolution of arms embargo monitoring by the United Nations (UN) Panels of Experts (PoEs) between 1996 and 2016. The study shows how dense sub-networks of experts spanning organisational boundaries, defined as trans-sectoral cliques, drove the emergence and mutation of practice on PoEs. Cliques shaped activity patterns within PoEs. They generated recognition for these activities from outside the PoEs through recognition cycles between 'inner' and 'outer' segments (i.e. between clique members who worked on PoEs and clique members who worked at other organisations) and they embedded the new arms monitoring practice in a knowledge system shared across institutions. The study reveals powerful mechanisms in the emergence of expert practice at IOs and points at the importance of dense networks and cross-organisational ties.