Men in Romania taking part in summit (5)
I believe the answer is:
troop
'men' is the definition.
(both can mean a group of soldiers)
'romania taking part in summit' is the wordplay.
'romania' becomes 'ro' (international vehicle registration code).
'taking part in' indicates putting letters inside.
'summit' becomes 'top' (synonyms).
'ro' inserted within 'top' is 'TROOP'.
'in' acts as a link.
(Other definitions for troop that I've seen before include "Flock; Scout group" , "Squad" , "Military group" , "A company, unit" , "Military unit" .)