Fish breed, we're told (4)
I believe the answer is:
rays
'fish' is the definition.
(I've seen this before)
'breed we're told' is the wordplay.
'breed' becomes 'raise' (I've seen this before).
'we're told' indicates a 'sounds like' (homophone) clue.
'raise' sounds like 'RAYS'.
(Other definitions for rays that I've seen before include "Flat fish" , "Beams - fish" , "Pencils of light" , "Fish with wing-shaped fins and long tails" , "Straight lines extending from a point" .)