Crossword Genius

Eager or wary? Source of joy is absent, sleep instead (7)

Ross

I believe the answer is:

zealous

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'eager' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

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(Other definitions for zealous that I've seen before include "Impassioned" , "Avidly enthusiastic" , "Showing great energy and enthusiasm" , "Highly enthusiastic" , "Very devoted to a cause" .)

I've seen this clue in The Hindu.
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