Arch they converted to a fish delivery business (8)
I believe the answer is:
hatchery
'fish delivery business' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'arch they converted' is the wordplay.
'converted' indicates an anagram.
'arch'+'they'='archthey'
'archthey' with letters rearranged gives 'HATCHERY'.
'to a' is the link.
Can you help me to learn more?
(Other definitions for hatchery that I've seen before include "[WOMB] of trout" , "Place for incubating fish eggs" , "Whence young" , "Where fish are bred" , "Fish-breeding establishment" .)