The most common and most likely answer to this crossword clue is the 5 letter word MELIC
M
E
L
I
C
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The straight part
We know that "Grass" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grass = melic.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 MELIC mean?
MELIC noun- Any of various grasses, of the genus Melica, from north temperate regions
- Of or pertaining to Greek lyric verse.