Test top of rainwater tank (10)
I believe the answer is:
challenger
'test top of rainwater tank' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for challenger that I've seen before include "Ill-fated space shuttle" , "One contesting a title" , "professor of literature" , "Claimant" , "Contender" .)