Gather a harvest (4)
I believe the answer is:
reap
'gather a harvest' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reap that I've seen before include "Save harvest, glean" , "'As you sow, so shall you . . . .'" , "Get -- derive" , "Harvest, garner" , "Glean or cut as harvest" .)