Be joined by a small number for party (5)
I believe the answer is:
beano
'for party' is the definition.
(I've seen this before)
'be joined by a small number' is the wordplay.
'joined by' is a charade indicator (letters next to each other).
'small number' becomes 'no' (abbreviation for 'number'; 'small' suggests a short form).
'be'+'a'+'no'='BEANO'
(Other definitions for beano that I've seen before include "Comic and informal get-together" , "Junket - children's comic" , "Function" , "Kids' comic" , "Rowdy jollification" .)