Display of top-grade river fish (5)
I believe the answer is:
array
'display' is the definition.
('array' can be a synonym of 'display')
'top-grade river fish' is the wordplay.
'top grade' becomes 'a'.
'river' becomes 'r'.
'fish' becomes 'ray' (ray is a kind of fish).
'a'+'r'+'ray'='ARRAY'
'of' is the link.
(Other definitions for array that I've seen before include "Matrix; finery" , "Line up" , "Orderly or impressive display" , "Imposing show or military order" , "Impressive exhibition" .)