Writes off what's surrounding opening in wood (9)

'writes off' is the definition.
Although both the answer and definition are verbs in their -s form, I don't see how they can define each other.

'surrounding opening in wood' is the wordplay.
I cannot really see how this works, but
'opening' could be 'mortise' (mortise is a kind of opening) and 'mortise' is present in the answer.
The remaining letters 'as' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'what's' acts as a link.

Can you help me to learn more?