Harry tethering spanner, perhaps? (9)

I believe the answer is:

tightener

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

'spanner perhaps?' is the definition.
I can't tell whether this definition defines the answer.

'harry tethering' is the wordplay.
'harry' indicates an anagram (harry can mean to attack or spoil).
'tethering' anagrammed gives 'TIGHTENER'.

Can you help me to learn more?

Want a hint initially instead of a full solution? Install my app