Instrument measuring flow provides three more (9)

I believe the answer is:

rheometer

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

'instrument' is the definition.
(I know that rheometer is a type of instrument)

'measuring flow provides three more' is the wordplay.
'three more' anagrams to 'RHEOMETER'.
But, I'm unsure how the anagram is indicated.

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