Angler pulled up unknown fish (4)
I believe the answer is:
dory
'fish' is the definition.
(dory is a kind of fish)
'angler pulled up unknown' is the wordplay.
'angler' becomes 'rod' (I can't explain this - if you can you should believe this answer much more).
'pulled up' says the letters should be written backwards.
'unknown' becomes 'y' (algebra).
'rod' written backwards gives 'dor'.
'dor'+'y'='DORY'
Can you help me to learn more?
(Other definitions for dory that I've seen before include "Sea-wolf" , "Fish and flat-bottomed boat" , "Fish for a small boat" , "Marine fish; rhymes with fable" , "Fish of mackerel family" .)