Boomerang lessons to catch fishes (6)
I believe the answer is:
angles
'fishes' is the definition.
(angling is a kind of fishing)
'boomerang lessons to catch' is the wordplay.
'to catch' says the answer is hidden in the clue (the answer is caught in the letters of the clue).
'ANGLES' can be found hidden inside 'boomerang lessons'.
(Other definitions for angles that I've seen before include "old Germanic tribespeople" , "Fishes for perspectives?" , "Attempts to catch one from school" , "Sharp corners" , "Figures formed by two lines meeting" .)