Began to write in dictionary (6)
I believe the answer is:
opened
'began' is the definition.
(opening is a kind of beginning)
'write in dictionary' is the wordplay.
'write' becomes 'pen' (synonyms).
'in' indicates putting letters inside.
'dictionary' becomes 'oed' (Oxford English Dictionary).
'pen' going within 'oed' is 'OPENED'.
'to' is the link.
(Other definitions for opened that I've seen before include "Turned key and unlocked" , "Undid, a parcel say, or began proceedings" , "Began - undone" , "Started, a meeting say" , "Unfurled" .)