Crossword Genius

Hell to write, breaking through dark as necessary (13)

Ross

I believe the answer is:

indispensable

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'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" .)

I've seen this clue in The Telegraph.
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