One of three in "To be or not to be" (4)
I believe the answer is:
iamb
'one of three in to be or not to be' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iamb that I've seen before include "Short-long metric foot" , "Metrical foot of one short, one long syllables" , "Part of a pentameter?" , "Short-long metrical foot" , "Metrical unit" .)