The most common and most likely answer to this crossword clue is the 6 letter word POLLEX
I've seen this crossword clue in The Guardian
P
O
L
L
E
X
More clues leading to the result POLLEX
Buddy explains!

The straight part
We know that "thumb" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thumb = pollex.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 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.