Crossword Genius

With energy, answer back (3)

Ross

I believe the answer is:

ago

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'back' is the definition.
('years back' means 'years ago')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ago that I've seen before include "Gone, past, since" , "Previously" , "Earlier, gone by" , "Back (in time)" , "Time past" .)

I've seen this clue in The Times.
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