To conclude, fish out what's inside (6)
I believe the answer is:
finish
'to conclude' is the definition.
(I've seen this before)
'fish out what's inside' is the wordplay.
'out' indicates putting letters inside.
'what's inside' becomes 'in'.
'fish' going around 'in' is 'FINISH'.
(Other definitions for finish that I've seen before include "Conclude, wind up" , "'End, close (6)'" , "Pefect" , "perfection" , "Polish, perhaps" .)