It's futile to consume a smaller amount (7)
I believe the answer is:
useless
'it's futile' is the definition.
(similar in meaning)
'consume a smaller amount' is the wordplay.
'consume' becomes 'use' (use can mean to consume or get through).
'a smaller amount' becomes 'less' (I've seen this in another clue).
'use'+'less'='USELESS'
'to' is the link.
(Other definitions for useless that I've seen before include "Without any worthwhile application" , "Good for nothing" , "Having no beneficial function" , "Ineffective" , "Of no benefit" .)