I agreed to bring up a kind of fish (3)
I believe the answer is:
koi
'a kind of fish' is the definition.
'koi' can be an answer for 'fish' (koi is a type of fish). I'm not sure about the 'a kind of' bit.
'i agreed to bring up' is the wordplay.
'agreed' becomes 'ok' (eg when responding to an offer).
'to bring up' shows that the letters should be reversed in order (I've seen 'brought up' mean this (in a down clue, letters go up)).
'i'+'ok'='iok'
'iok' in reverse letter order is 'KOI'.
Can you help me to learn more?
(Other definitions for koi that I've seen before include "Kind of decorative carp" , "Japanese type of carp" , "Carp; sounds like shy" , "sort of fish" , "Ornamental carp variety" .)