In days gone by, I pocketed commission (4)
I believe the answer is:
agio
'commission' is the definition.
(I've seen this before)
'in days gone by i pocketed' is the wordplay.
'in days gone by' becomes 'ago' (both can refer to a long time in the past).
'pocketed' indicates putting letters inside.
'ago' going around 'i' is 'AGIO'.
(Other definitions for agio that I've seen before include "Money-changing" , "Changing money" , "Transfer fee" , "charge when money changes hands" , "Discount" .)