Large amount in drain, a lot wasted (9)
I believe the answer is:
trainload
'large amount' is the definition.
'trainload' can be an answer for 'amount' (trainload is a kind of amount). I'm not sure about the 'large' bit.
'drain a lot wasted' is the wordplay.
'wasted' indicates anagramming the letters.
'drain'+'a'+'lot'='drainalot'
'drainalot' is an anagram of 'TRAINLOAD'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for trainload that I've seen before include "Express content" , "all those expecting to be late?" , "people aboard" , "Many commuters" .)