But a lot's wrong if it's less than the full amount (8)
I believe the answer is:
subtotal
'amount' is the definition.
(I know that subtotal is a type of amount)
'but a lot's wrong' is the wordplay.
'wrong' indicates an anagram.
'but'+'a'+'lots'='butalots'
'butalots' anagrammed gives 'SUBTOTAL'.
'if it's less than the full' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for subtotal that I've seen before include "Part-sum in calculation" , "Sum of a set of figures out of a larger group" , "Blast out (anag.)" , "Work out in column, say" , "Part of an addition" .)