Hint: gloom transformed by this? (9)
I believe the answer is:
moonlight
'this?' is the definition.
The definition suggests a singular noun which matches the answer.
'hint gloom transformed' is the wordplay.
'transformed' indicates anagramming the letters.
'hint'+'gloom'='hintgloom'
'hintgloom' with letters rearranged gives 'MOONLIGHT'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for moonlight that I've seen before include "Do extra work" , "Beethoven composition for piano" , "Burn the midnight oil" , "do an extra job" , "name for sonata" .)