The greatest amount (4)
I believe the answer is:
most
'the greatest amount' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for most that I've seen before include "Kind of commercial pact where two countries agree to treat each other as well as any other" , "In the highest degree" , "Greatest in amount or degree" , "Extremely, very" , "Almost all" .)