What’s hard in writing down cipher (7)
I believe the answer is:
nothing
'cipher' is the definition.
('nothing' can be a synonym of 'cipher')
'what's hard in writing down' is the wordplay.
'what's hard' becomes 'h'.
'in' is an insertion indicator.
'writing down' becomes 'noting' (noting is a kind of writing down).
'h' placed within 'noting' is 'NOTHING'.
(Other definitions for nothing that I've seen before include "Something sweet" , "Zero, nil" , "A void" , "Duck" , "One of those sweet sayings" .)