... on account of us backing cases in competition (7)
I believe the answer is:
because
'on account of' is the definition.
(I've seen this before)
'us backing cases in competition' is the wordplay.
'backing' is a charade indicator (letters next to each other).
'cases' becomes 'ca' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'competition' becomes 'bee' (I've seen this before).
'us' after 'ca' is 'caus'.
'caus' inserted within 'bee' is 'BECAUSE'.
Can you help me to learn more?
(Other definitions for because that I've seen before include "For the reason that" , "Since" , "Owing to" , "On account of" , "Thanks to this" .)