Form 1040 amount (6)
I believe the answer is:
income
'form 1040 amount' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for income that I've seen before include "Money received as wages, interest etc" , "Earned or unearned revenue." , "What do you make" , "Pay earned" , "Money gained from work or investment" .)