Fish in Test dropping end of fly (5)
I believe the answer is:
trout
'fish' is the definition.
(I know that trout is a type of fish)
I can't explain the remainder of the clue.
(Other definitions for trout that I've seen before include "air" , "Unattractive old woman" , "Tickled fish?" , "Schubert Quintet?" , "unpleasant old woman" .)