The most common and most likely answer to this crossword clue is the 6 letter word POLLEX
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More clues leading to the result POLLEX
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The cryptic/wordplay part
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What does POLLEX mean?
POLLEX noun- The thumb; the first, or preaxial, digit of the forelimb, corresponding to the hallux in the hind limb. In birds, the pollex is the joint which bears the alula or bastard wing.