I agree to pack additional line for this fish (5,3)
I believe the answer is:
moray eel
'this fish' is the definition.
(moray eel is a kind of fish)
'i agree to pack additional line' is the wordplay.
'i agree' becomes 'aye' (I've seen this in another clue).
'to pack' indicates putting letters inside.
'additional line' becomes 'morel' (I can't explain this - if you can you should believe this answer much more).
'aye' going within 'morel' is 'MORAY EEL'.
'for' acts as a link.
Can you help me to learn more?
(Another definition for moray eel that I've seen is " Predatory fish".)