Weaken nun - wise move (7)

I believe the answer is:

unsinew

I'm a little stuck... Click here to teach me about this clue!

Here is my best explanation:

'weaken' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'nun wise move' is the wordplay.
'move' indicates an anagram.
'nun'+'wise'='nunwise'
'nunwise' anagrammed gives 'UNSINEW'.

Can you help me to learn more?