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 can't understand how one could define the other.

'surrounding opening in wood' is the wordplay.
I cannot quite understand how this works, but
'opening' could be 'mortise' (mortise is a kind of opening) and 'mortise' is found in the answer.
The remaining letters 'as' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'what's' is the link.

Can you help me to learn more?