I set about some fish (6)
I believe the answer is:
plaice
'some fish' is the definition.
(plaice is a kind of fish)
'i set about' is the wordplay.
I cannot really understand how this works, but
'i' could be 'a' and 'a' is found within the answer.
'about' could be 'c' (abbreviation for circa) and 'c' is located in the answer.
The remaining letters 'plie' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for plaice that I've seen before include "Location of the fish" , "Brown flatfish" , "Flat fare" , "Important food fish" , "Flatfish; sounds like put" .)