Many invited to a kind of ball (6)
I believe the answer is:
masked
'to a kind of ball' is the definition.
(I've seen this in another clue)
'many invited' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'invited' becomes 'asked' (I've seen this in another clue).
'm'+'asked'='MASKED'
(Other definitions for masked that I've seen before include "With covered face" , "Disguised, covered" , "Blocked out, disguised" , "Unseen" .)