Anniversary organised in canteen, attended by fifty (10)
I believe the answer is:
centennial
'anniversary' is the definition.
(centennial is a kind of anniversary)
'organised in canteen attended by fifty' is the wordplay.
'organised' is an anagram indicator.
'attended by' says to put letters next to each other.
'fifty' becomes 'l' (roman numeral).
'in'+'canteen'='incanteen'
'incanteen' is an anagram of 'centennia'.
'centennia'+'l'='CENTENNIAL'
(Other definitions for centennial that I've seen before include "Hundredth anniversary" , "special anniversary" .)