Practise economy, extremely likely without effect (9)
I believe the answer is:
uselessly
'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 give a lot more credence to this answer).
'extremely' says to hollow out the word (remove centre letters) (letters on the extremes of the word are used).
'likely' with its middle taken out is 'ly'.
'useless'+'ly'='USELESSLY'
Can you help me to learn more?
(Another definition for uselessly that I've seen is " in vain".)