Knowing ships throw away river fishing gear (3,5)

'knowing ships throw away river fishing gear' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'knowing ships throw away river fishing gear' is the wordplay.
I cannot really understand how this works, but
'knowing' could be 'fly' and 'fly' is present in the answer.
'fishing gear' could be 'lines' (I've seen this in another clue) and 'lines' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...

Can you help me to learn more?