Knots tethering spanner at times (9)

I believe the answer is:

tightener

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

'spanner at times' is the definition.
I can't tell whether this defines the answer.

'knots tethering' is the wordplay.
'knots' indicates anagramming the letters (I've seen 'knot' mean this (letters are tangled up)).
'tethering' with letters rearranged gives 'TIGHTENER'.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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