Extremely useful information? (3)
I believe the answer is:
tip
'information?' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tip that I've seen before include "lots of rubbish" , "Piece of useful advice" , "Empty" , "Topple; advice" , "Bonus" .)