River backing a star (4)

I believe the answer is:

nova

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'star' is the definition.
(I know that nova is a type of star)

'river backing a' is the wordplay.
'river backing' becomes 'nov' (I can't explain this - if you can you should give a lot more credence to this answer).
'nov'+'a'='NOVA'

Can you help me to learn more?

(Other definitions for nova that I've seen before include "Sudden light increase" , "Kind of star" , "sun" , "Star showing a sudden, brief increase in luminosity" , "Star that suddenly becomes brighter for a time" .)

I've seen this clue in The Irish Independent.

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