Vain advice not to consume so much (7)
I believe the answer is:
useless
'vain' is the definition.
(I've seen this in another clue)
'advice not to consume so much' is the wordplay.
'advice not' becomes 'les' (I can't justify this - if you can you should believe this answer much more).
'to' says to put letters next to each other (I've seen this in other clues).
'consume' becomes 'use' (use can mean to consume or get through).
'much' means to remove the last letter (much but not all of the word).
'so' with its last letter removed is 's'.
'les' after 'use' is 'useles'.
'useles'+'s'='USELESS'
Can you help me to learn more?
(Other definitions for useless that I've seen before include "Not practical or constructive" , "Worth zilch" , "Is Sue less impractical?" , "Good for nothing" , "Incompetent" .)