Fishy business in 3 down is a growing enterprise (5,4)
I believe the answer is:
trout farm
'fishy business' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for trout farm that I've seen before include "Sort of aquaculture" , "Where fish are bred" , "Fish-rearing concern" , "where fish are found" , "fishy business" .)