Clue: Art form involved with origins of the underworld and source of rainbows? (5,4)
Answer: TROUT FARM
Explanation: Definition: source of rainbows?
***(art form t u) **[anag:*involved*] where T U are the [*origins of*] T(he) + U(nderworld) 
 
 
*The rainbows referred to in the clue are rainbow trout.