Hell to write, breaking through dark as necessary (13)
I believe the answer is:
indispensable
'necessary' is the definition.
(associated in meaning)
'hell to write breaking through dark as' is the wordplay.
'hell' becomes 'dis' (I've seen this before).
'to write' becomes 'pen' ('pen' can be a synonym of 'write').
'breaking' indicates putting letters inside.
'through' becomes 'in'.
'dark as' becomes 'sable' (I can't justify this - if you can you should give a lot more credence to this answer).
'dis'+'pen'='dispen'
'in'+'sable'='insable'
'dispen' placed within 'insable' is 'INDISPENSABLE'.
Can you help me to learn more?
(Other definitions for indispensable that I've seen before include "Vitally necessary" , "Absolutely necessary" , "Essential" .)