This could be down or across! (11)
I believe the answer is:
preposition
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'be down or across' is the wordplay.
I cannot really see how this works, but
'be' could be 'sit' (sitting is a kind of being) and 'sit' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'could' acts as a link.
Can you help me to learn more?
(Other definitions for preposition that I've seen before include "One is in" , "Possibly in" , ""Before" or "after", say" , "Part of speech" , "Word like after or for (grammar)" .)