Cracks very large case for writer (5)
I believe the answer is:
opens
'cracks' is the definition.
(I've seen this in another clue)
'very large case for writer' is the wordplay.
'very large' becomes 'os' (abbreviation for oversize).
'case for' is an insertion indicator.
'writer' becomes 'pen' (something which writes).
'os' enclosing 'pen' is 'OPENS'.
(Other definitions for opens that I've seen before include "Exposes, begins" , "Provides access to" , "releases" , "competitions welcoming all-comers" , "Allows access" .)