The most common and most likely answer to this crossword clue is the 5 letter word EAGER
I've seen this crossword clue in The Guardian
E
A
G
E
R
More clues leading to the result EAGER
- Keen to be a long time in Her Majesty's embrace?
- Wholehearted bore needing an audience
- Showing enthusiasm, the Queen embraces maturity
- Avid listener for example back inside
- Keen, agree to dance
- . Somewhat like a gerbil or like a beaver
- Impatiently desiring drink getting euro at outset for pounds
- Keen listener describing good ending to programme
- Time to stop queen being keen
- Enthusiastic but not sure about time
Buddy explains!

The straight part
We know that "Zealous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case zealous = eager.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does EAGER mean?
EAGER adjective- Sharp; sour; acid.
- Sharp; keen; bitter; severe.
- Desirous; keen to do or obtain something.
- Brittle; inflexible; not ductile.
- Not employing lazy evaluation; calculating results immediately, rather than deferring calculation until they are required.
- A tidal bore