Gathering of crops is most tough with little volume brought in for days (7)
I believe the answer is:
harvest
'gathering of crops' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
(Other definitions for harvest that I've seen before include "A season's yield, crop" , "Process of gathering mature crops" , "The fruits of tillage" , "Yield from plants in a single growing season" , "Gather in (a crop)" .)