Fishes in corners (6)
I believe the answer is:
angles
This clue is a double definition.
'fishes' is the first definition.
(angling is a kind of fishing)
'corners' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for angles that I've seen before include "Spaces between lines that meet" , "Points where two lines intersect" , "Spaces between intersecting lines" , "1 across have three" , "Germanic people who settled in Britain in the fifth century" .)