Compose beginning of wedding ceremony (5)
I believe the answer is:
write
'compose' is the definition.
(I know that compose can be written as write)
'beginning of wedding ceremony' is the wordplay.
'beginning of' says to take the initial letters.
'ceremony' becomes 'rite' (rite is a kind of ceremony).
The first letter of 'wedding' is 'w'.
'w'+'rite'='WRITE'
(Other definitions for write that I've seen before include "Inscribe" , "Compose (music)" , "Jot" , "Put pen to paper" , "Create (manuscript)" .)