Agent gathering a harvest (4)
I believe the answer is:
reap
'harvest' is the definition.
(I know that harvest can be written as reap)
'agent gathering a' is the wordplay.
'agent' becomes 'rep' (short for representative).
'gathering' means one lot of letters goes inside another (gather can mean to acquire or bring in).
'rep' enclosing 'a' is 'REAP'.
(Other definitions for reap that I've seen before include "Get -- derive" , "Harvest, garner" , "Gather 23dn" , "Use a scythe" , "Glean or cut as harvest" .)