The most common and most likely answer to this crossword clue is the 8 letter word GEMINATE
G
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The straight part
We know that "Pair" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pair = geminate.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
GEMINATE noun- A doubled or repeated letter or speech sound.
- To arrange in pairs.
- To occur in pairs.
- Forming a pair.
- Of a consonant, pronounced longer and considered as being doubled