Piece of fish I dropped: that's done it (8)
I believe the answer is:
finished
'done it' is the definition.
The answer and definition can be both to do with contact as well as being past participle verbs.
Perhaps you can see a link between them that I don't see?
'piece of fish i dropped' is the wordplay.
'piece of fish' becomes 'fin' (fish have fins).
'dropped' becomes 'shed' ('shed' can be a synonym of 'drop').
'fin'+'i'+'shed'='FINISHED'
'that's' is the link.
Can you help me to learn more?
(Other definitions for finished that I've seen before include "wrapped up" , "Brought to an end" , "Done, over, ended" , "dressed" , "Complete" .)