Knock up a note before ten? It can be true (7)
I believe the answer is:
paradox
'it can be true' is the definition.
The definition suggests a singular noun which matches the answer.
'knock up a note before ten?' is the wordplay.
'knock' becomes 'rap' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'note before' becomes 'do' (musical note, as in do-re-mi. I am not sure about the 'before' bit.).
'ten?' becomes 'X' (Roman numeral).
'rap' back-to-front is 'par'.
'par'+'a'+'do'+'x'='PARADOX'
Can you help me to learn more?
(Other definitions for paradox that I've seen before include "A statement that seems to contradict itself" , "Statement seeming to be self-contradictory" , "Seemingly inconsistent statement" , "Absurd or self-contradictory statement" , "Seeming contradiction" .)