Crossword Genius

Erstwhile partner works to return bonus (5)

Ross

I believe the answer is:

extra

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'bonus' is the definition.
(thesaurus)

'erstwhile partner works to return' is the wordplay.
'erstwhile' becomes 'ex'.
'partner works' becomes 'art' (I can't justify this - if you can you should give a lot more credence to this answer).
'to return' says the letters should be written backwards.
'art' backwards is 'tra'.
'ex'+'tra'='EXTRA'

Can you help me to learn more?

(Other definitions for extra that I've seen before include "comes to 16 [TEST MATCH]" , "Bolt-on" , "Not a standard item" , "on top" , "Minor actor in crowd scenes" .)

I've seen this clue in The Independent.
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