Catching the fish that form shoals (6)
I believe the answer is:
gather
'catching the fish that form shoals' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'catching the fish' is the wordplay.
'catching' means one lot of letters goes inside another.
'fish' becomes 'gar' (type of fish).
'the' inserted within 'gar' is 'GATHER'.
Can you help me to learn more?
(Other definitions for gather that I've seen before include "Come/bring together" , "Collect or asemble" , "Collect together in one place" , "Come together for the rag" , "... will proverbially do this" .)