Romeo joins a friend at the gathering (5)
I believe the answer is:
rally
'the gathering' is the definition.
(rally is a kind of gathering)
'romeo joins a friend' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'joins' says to put letters next to each other.
'a friend' becomes 'ally' (an ally is a friend or supporter).
'r'+'ally'='RALLY'
'at' acts as a link.
(Other definitions for rally that I've seen before include "motoring event" , "Arouse for action" , "Exchange of shots (tennis)" , "several shots" , "Regain health" .)