Beginning to open very large letter (5)
I believe the answer is:
omega
'letter' is the definition.
(I know that omega is a type of letter)
'beginning to open very large' is the wordplay.
'beginning to' indicates taking the first letters.
'very large' becomes 'mega' (I've seen this in another clue).
The initial letter of 'open' is 'o'.
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "Art of story-telling by body movement" , "Last (Gk. letter)" , "character in Greek literature?" , "letter from abroad" , "completes a series" .)