Catching the fish that form shoals (6)
I believe the answer is:
gather
'catching the fish that form shoals' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'catching the fish' is the wordplay.
'catching' is an insertion indicator.
'fish' becomes 'gar' (gar is a kind of fish).
'the' placed inside '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" .)