Practise economy, extremely likely without effect (9)

I believe the answer is:

uselessly

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

'without effect' is the definition.
I can't judge whether this defines the answer.

'practise economy extremely likely' is the wordplay.
'practise economy' becomes 'useless' (I can't explain this - if you can you should believe this answer much more).
'extremely' suggests removing the centre (only the extremes of the word are used).
'likely' with its middle removed is 'ly'.
'useless'+'ly'='USELESSLY'

Can you help me to learn more?

(Another definition for uselessly that I've seen is " in vain".)

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