Angler's instrument brought up unknown quantity of fish (4)
I believe the answer is:
dory
'fish' is the definition.
(dory is a kind of fish)
'angler's instrument brought up unknown quantity' is the wordplay.
'angler's instrument' becomes 'rod' (a rod might be used when angling).
'brought up' says the letters should be written backwards (in a down clue, letters go up).
'unknown quantity' becomes 'y' (algebra).
'rod' in reverse letter order is 'dor'.
'dor'+'y'='DORY'
'of' acts as a link.
(Other definitions for dory that I've seen before include "Marine fish; rowing boat" , "Marine fish; rhymes with fable" , "Fish of mackerel family" , "Sea-wolf" , "Golden-yellow fish" .)