Part of betrothal festivities (4)
I believe the answer is:
half
'part' is the definition.
(I know that half is a type of part)
'of betrothal festivities' is the wordplay.
'of' indicates a hidden word.
'HALF' is hidden in the letters of 'betrothal festivities'.
(Other definitions for half that I've seen before include "Wife the better one (joc.)" , "Fifty percent" , "50 per cent" , "Short distance - steal!" , "Fifty per cent" .)