The most common and most likely answer to this crossword clue is the 8 letter word SYNTAGMA
S
Y
N
T
A
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A
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The straight part
We know that "Part of grammatical structure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of grammatical structure = syntagma.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.
Crossword Answer definitions
SYNTAGMA noun- A constituent segment within a text, such as a word or a phrase that forms a syntactic unit.
- An arrangement of units that together bears a meaning.
- (history) A Macedonian phalanx fighting formation consisting of 256 men with long spears (sarissae).