Breed fish one's caught (5)
I believe the answer is:
raise
'breed' is the definition.
(I've seen this in another clue)
'fish one's caught' is the wordplay.
'fish' becomes 'rae' (I can't explain this - if you can you should give a lot more credence to this answer).
'one's' becomes 'I's' (Roman numeral).
'caught' means one lot of letters goes inside another.
'rae' enclosing 'is' is 'RAISE'.
Can you help me to learn more?
(Other definitions for raise that I've seen before include "Lift, elevate" , "Inflate" , "Pick up" , "jack up" , "Cultivate by growing" .)